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A    MANUAL 


OF 


PRACTICAL     NORMAL 
H  ISTOLOGY 


BY 


T.    MITCHELL    PRUDDEN,     M.  D. 

DIRECTOR  OF   THE  PHYSIOLOGICAL  AND  PATHOLOGICAL  LABORATORY  OF   THE  ALUMNI 

ASSOCIATION     OF     THE     COLLEGE     OF     PHYSICIANS     AND     SURGEONS,    N.    Y.  ; 

LECTURER  ON  NORMAL  HISTOLOGY  IN  YALE  COLLEGE  ;  PATHOLOGIST 

TO  THE   MANHATTAN    EYE   AND    EAR   HOSPITAL. 


NEW     YORK 

G.    P.    PUTNAM'S    SONS 

27   AND    29   WEST    23D    STREET 
l8Sl 


Copyright  by 
G.  P.  PUTNAM'S  SONS 

I88l 


Press  fif 

G.  P%  Putnam's  Sons 

New  York 


PREFACE. 


This  book  has  been  prepared  for  the  use  of  those 
students  and  practitioners  of  medicine,  who  with  a 
limited  amount  of  time  at  their  disposal,  wish  to 
acquaint  themselves  in  a  practical  way  with  Normal 
Histology.  It  is  especially  designed  for  those  who 
study  the  science  in  classes,  with  an  instructor 
in  a  laboratory;  but  the  technical  .procedures  are 
described  with  sufficient  fulness  for  the  needs  of 
those  who  are  obliged  to  pursue  the  study  by  them- 
selves. 

The  method  adopted  is  to  give  a  brief  description 
of  the  tissues  and  organs  in  appropriate  sequence, 
following  each  description  with  an  account  of  the 
way  in  which  the  structures  described  may  be  dem- 
onstrated. The  descriptions  were  written  for  the 
most  pait  at  the  microscrope  table,  with  the  prepar- 
ations made  by  the  methods  recommended,  under 
the  eye  of  the  writer,  so  that  it  is  believed  that  the 
student  will  have  no  difficulty  in  verifying  them. 

Too  much  stress  cannot  be  laid  upon  the  neces- 
sity of  each  student  making  outline  sketches  of  all 


IV  .  PREFACE. 

but  the  more  complicated  structures  examined,  and 
for  this  provision  has  been  made  in  the  book.  It  is 
not  to  be  expected  that  epitomized  descriptions  of 
structures  as  elaborate  as  are  many  of  those  with 
which  we  have  to  deal  in  Human  Histology,  will  be 
in  all  cases  perfectly  clear  and  intelligible  without 
the  aid  of  plates  ;  but  the  specimens  which  the  stu- 
dent prepares,  and  the  sketches  from  them  which 
he  makes,  will  make  good,  it  is  hoped,  the  lack  of 
illustration  in  the  text.  Indeed,  the  more  critical 
examination  which  accurate  sketching  requires,  as 
well  as  the  facility  which  this  exercise  cultivates, 
will  enlarge  the  achievements  of  such  a  course  of 
study  beyond  the  acquirement  of  a  knowledge  of 
this  theme  alone,  so  as  to  embrace  a  valuable  train- 
ing of  the  eye  and  hand. 

This  book  is  not  designed  to  take  the  place  of 
more  elaborate  treatises  on  this  subject ;  nor  is  it 
written  with  the  design  of  fostering  the  deplorably 
wide-spread  tendency  among  medical  students,  to 
be  content  with  the  barest  smattering  of  those 
branches  which  are  not  in  the  most  evident  manner 
"  practical."  On  the  contrary,  where  time  permits, 
collateral  reading  and  additional  practical  work  are 
most  urgently  recommended.  But  the  necessity  for 
improvement  in  medical  education,  which  is  expres- 
sing itself  in  the  medical  colleges  of  this  country, 
especially  in  the  establishment  of  laboratories  and 
practical  courses  of    instruction,  is,    unfortunately, 


PREFACE.  V 

not  yet  sufficiently  deeply  felt  as  to  have  led  to  the 
general  lengthening  of  the  period  of  undergraduate 
study  ;  so  that  very  little  time  is  usually  at  the  dis- 
posal of  medical  students  for  collateral  reading,  or 
for  the  pursuit  of  elaborate  practical  investigations. 
It  is  desirable,  moreover,  since  the  laboratory  time 
itself  is  usually  limited,  to  occupy  as  little  of  it  as 
may  be,  in  oral  descriptions  of  tissues  and  methods. 

It  is  these  considerations  which  seem  to  justify  the 
addition  of  another  to  the  long  list  of  elementary 
text-books. 

There  are  many  points  in  this,  as  in  every  develop- 
ing science,  which  are  still  unsettled — opinion  in  re- 
gard to  them  changing  or  being  modified  as  new 
facts  and  investigations  are  recorded.  These  have 
been  treated,  for  the  most  part,  very  briefly  in  the 
text,  it  being  left  for  the  supplementary  oral  instruc- 
tion to  enlarge  upon  and  explain  them,  as  the  light 
thrown  upon  each  by  new  researches  may  seem  to 
require. 

In  the  simpler  form  of  "Notes  on  the  Practical 
Course  in  Normal  Histology  "  the  substance  of  this 
book  has  been  in  use  for  two  years  in  the  laboratory 
of  the  Alumni  Association  of  the  College  of  Phy- 
sicians and  Surgeons,  and  it  has  been  found  that 
with  some  preliminary  preparation  of  tissues  by  the 
instructor,  the  subject  essentially  as  presented  here, 
can  be  embraced  in  a  course  of  forty  lessons  of  about 
two  hours  each. 


VI  PREFA  CE. 

The  writer  wishes,  in  conclusion,  to  express  his 
sincere  thanks  to  Prof.  Francis  Delafield,  to  whose 
wise  counsel  and  unwearied  assistance  in  many  mat- 
ters requiring  a  wider  experience  than  his  own,  he 
is  greatly  indebted.  T.  M.  P. 


Laboratory  of  the  Alumni  Associa- 
tion of  the  College  of  Physicians 
and  Surgeons. 

New  York,  May,  1881. 


CONTENTS 


PAGE 

INTRODUCTION              .                   -                   -                  -                  .  i 

I THE    CELL    IN    GENERAL                        -                  -  I  7 

II — CONNECTIVE    TISSUE                     -                  -  27 

III — EMBRYONAL  AND    MUCOUS    TISSUE — FAT  TIS- 
SUE—  RETICULAR    CONNECTIVE    TISSUE  48 

IV CARTILAGE BONE — TEETH              -                  -  56 

V BLOOD    AND    LYMPH                      -                   -  80 

VI — MUSCULAR    TISSUE                -                  -                  •  9 1 

VII NERVE-TISSUE                 -                  -                  -                  -  103 

VIII — BLOOD-VESSELS — LYMPHATIC    VESSELS        -  II9 

IX — LYMPH-NODES — SPLEEN             -                  -  1 29 

X THE    GASTRO-INTESTINAL    CANAL                   -  143 

XI SUBMAXILLARY    GLAND LIVER             -                   -  155 

XII SUPRA-RENAL    CAPSULES— THYROID    GLAND  164 

XIII THE    RESPIRATORY    APPARATUS             -                   -  168 

XIV — THE    KIDNEY            -                  -                  -                  -  1 78 

XV THE    GENERATIVE    ORGANS     -                  -                  -  1 88 

XVI — THE    CENTRAL    NERVOUS    SYSTEM                   -  211 

XVII — THE    SKIN    AND    ITS    ADNEXA                   -                  -  220 

XVIII — THE    EYE                    ....  232 


INTRODUCTION. 

GENERAL    METHODS    FOR    PRESERVING    TISSUES    AND    PRE- 
PARING   THEM    FOR    STUDY. 

Animal  tissues  must  conform  to  certain  physical 
conditions  before  they  can  be  subjected  to  a  satis- 
factory microscopical  examination.  Portions  of 
them  subjected  to  study  must  be  sufficiently  thin 
to  allow  the  light  to  pass  readily  through  them,  and 
transparent  enough  to  permit  the  determination  of 
the  form,  character,  and  relations  of  their  structural 
elements.  At  the  same  time  the  refractive  power 
of  the  different  elements  should  not  be  too  nearly 
alike,  since  upon  differences  in  this  respect,  the 
form  and  characters  which  microscopical  objects 
present  to  the  eye  are  largely  dependent ;  or,  in 
case  they  are  so,  the  different  elements  must  be 
rendered  visible  by  staining  them  with  coloring 
agents. 

Certain  tissues  naturally  undergo  rapid  changes 
of  structure  after  death  ;  these  are  to  be  prevented 
by  the  use  of  preservative  agents.  Some  are  too 
soft  to  permit  the  preparation  of  thin  sections,  and 
must  be  hardened  ;  others  are  too  hard  and  must  be 
softened.    In  some  specimens  one,  in  others  another. 


2  NORMAL  HISTOLOGY. 

structural  feature  is  to  be  brought  into  prominence. 
All  of  these  indications  in  the  histological  technique 
are  to  be  met  in  such  a  way  as  to  leave  the  struc- 
tures under  investigation  in  as  natural  a  form  as 
possible.  Finally,  specimens  suitably  prepared  for 
examination,  are  in  many  cases,  to  be  rendered  per- 
manent for  future  reference  and  study. 

We  will  now  consider  briefly  some  of  the  methods 
by  which  these  indications  may  be  fulfilled.  Most 
histological  specimens  are  laid  in  some  enclosing 
fluid  medium,  on  a  glass  plate  and  covered  with  a 
thin  slip  of  glass,  before  being  brought  upon  the 
microscope.  One  of  the  simplest  methods  of  study- 
ing tissues  is  to  place  them,  when  quite  fresh  and 
after  they  are  reduced  to  a  condition  of  suitable 
tenuity,  on  a  slide  with  some  fluid  which  alters  their 
physical  condition  but  little  or  not  at  all,  or  at  least 
very  slowly,  and  examine  them  at  once.  Such  a 
fluid  is  called  an  indifferent  fluid,  and  for  most  pur- 
poses a  dilute  solution  of  common  salt,  one-half  to 
three-quarters  per  cent.,  answers  very  well. 

The  examination  of  fresh  tissues  is  very  important, 
not  only  because  it  enables  us  to  study  the  vital 
phenomena  in  certain  elements,  but  because  we  are 
thus  enabled  by  comparison  to  determine  the 
amount  of  change  which  tissues  undergo  when  pre- 
pared by  more  elaborate  methods.  Still  it  is  in 
many  respects  unsatisfactory.  In  the  first  place,  it 
is  not  always  easy  to  procure  fresh  tissues  for  every 


INTRODUCTION.  3 

observation,  and  even  in  an  indifferent  flui  ;es 

sooner  or  later  undergo  considerable  alterations,  so 
that  they  cannot  be  permanently  preserved.  A 
still  more  important  deficiency  in  this  method  is  the 
lack  of  distinctness  in  structural  details  which  it  in- 
volves. Most  of  the  fresh  animal  tissues  are  nearly 
transparent  in  thin  pieces,  and  their  structural  ele- 
ments have  so  nearly  the  same  refractive  power, 
that  we  see  through  them,  but  do  not  see  them  ;  or, 
if  we  do  see  them,  it  is  not  with  that  definiteness 
which  our  purposes  demand.  Xow  these  difficulties 
are  usually  met  by  the  use  of  agents  which  harden 
and  preserve  the  tissues  and  at  the  same  time  ren- 
der the  details  of  their  structure  visible,  by  changing 
the  refractive  power  of  one  or  other  of  their  ele- 
ments ;  or,  we  employ  certain  coloring  agents, 
which,  being  taken  up  with  different  degrees  of 
avidity  by  different  parts,  assist  in  the  recognition 
of  details  by  differences  in  color  :  or,  such  agents  are 
used  as  both  harden  and  stain  at  once  ;  or,  finally 
which  is  the  most  common  method,  we  employ  two 
or  more  of  the  different  classes  of  agents,  one  after 
the  other.  We  shall  consider  here  only  some  of  the 
most  common  and  useful  of  these  agents. 

HARDENING    AND    PRESERVATIVE    AGENTS. 

Alcohol  is  one  of  the  most  valuable  of  these.     It 
causes    a    considerable   shrinkage   of    most  tissu. 
partly  by  the  withdrawal  of  water  from  them,  and, 


4  NORMAL  HISTOLOGY. 

like  many  of  these  fluids,  precipitates  certain  of 
their  albuminoid  constituents,  thus  diminishing 
their  transparency.  It  is  in  general  to  be  used  at 
first  diluted  with  one-third  water,  and  after  the  tis- 
sues have  lain  for  24  hours  in  this,  they  are  trans- 
ferred to  strong  alcohol  in  which  they  may  be  pre- 
served indefinitely.  Bits  of  tissue  to  be  preserved 
in  alcohol,  as  in  other  hardening  agents,  should  be 
quite  small,  not  larger,  as  a  rule,  than  1  or  2  cms.  on 
a  side,  and  the  quantity  of  fluid  should  be  abun- 
dant. Certain  structures  are  best  preserved  by 
plunging  them  at  once  into  strong  alcohol. 

Aqueous  solutions  of  chromic  acid  and  of  potas- 
sium and  ammonium  bicliromate  and  of  the  neutral 
chromatc  of  ammonium  arc  much  used  to  preserve 
and  harden  tissues,  and  many  structures  are  more 
perfectly  preserved  in  these  fluids  than  in  alcohol. 
The  hardening  proceeds  more  slowly  than  in  alco- 
hol, and  the  structures  do  not  shrink  as  much.  It 
is  a  common  practice  to  commence  the  hardening 
with  one  of  the  chromic  fluids  and  complete  it  with 
alcohol,  the  former  being  thoroughly  washed  out 
before  immersion  in  the  latter.  Specimens  should 
not  remain  too  long  in  chromic  fluids  since  they  are 
apt  to  become  brittle.  An  immersion  of  from  2-3 
weeks  usually  suffices,  especially  if  alcohol  is  to  be 
used  afterward.  Chromic  acid  may  be  used  in  solu- 
tions of  from  \  to  \  per  cent.,  potassium  and  am- 
monium bichromate  in  2  per  cent.,  and  the  neutral 
ammonium  chromate  in  5-per-cent.  solution. 


INTRODUCTION,  5 

A  very  useful  preservative  solution  is  Midlers 
Fluid,  consisting  of 

Sodium   sulphate,  i 

Potassium  bichromate,         2 
Water,  100 

Chromic  acid  renders  the  lime  salts  in  bone  soluble, 
and  is  often  used  to  soften  them  in  preparation  for 
section-cutting.  .  / 

Picric  Acid. — This  agent  while  hardening,  pre- 
serves the  structure  of  many  tissues  very  perfectly, 
and  at  the  same  time  stains  them  yellow.  It  is  usu- 
ally necessary  to  complete  the  hardening  with  alco- 
hol. For  the  decalcification  of  bone  it  is  one  of  the 
best  of  agents,  although  it  acts  very  slowly.  It  is 
used  in  saturated  aqueous  solution. 

Osmic  Acid. — This  substance  has  the  power  of 
fixing  and  hardening  the  tissue  elements  in  a  nearly 
normal  form,  and  is  one  of  the  most  valuable  of  this 
class  of  agents.  It  gives  tissues  a  gray  or  brown  ap- 
pearance, and  stains  fat  and  certain  allied  substances 
deep  black.  It  is  used  in  one-per-cent.  aqueous 
solution.  The  tissue  should  be  quite  fresh  when 
immersed  in  it,  and,  as  a  rule,  should  remain  for 
twenty-four  hours.  Specimens  hardened  in  osmic 
acid  commonly  become  quite  granular  and  dark 
after  a  time. 

Preservative  fluids  are  sometimes  brought  into 
more   direct   contact  with   the  tissues,  by  injecting 


6  NORMAL  HISTOLOGY. 

them  into  the  blood-vessels  of  the  part  before  cut- 
ting it  in  pieces  ;  or  they  may  be  driven  directly 
into  the  interstices  of  the  tissue,  by  means  of  a 
small  syringe  with  a  sharp-pointed  canula ;  this  is 
called  interstitial  injection. 

Indications  as  to  which  of  these  agents  are  best 
adapted  for  the  preservation  of  different  tissues,  and 
the  more  exact  details  of  the  methods  of  employing 
them,  will  be  given  as  we  proceed  with  our  practical 
study. 

STAINING    AGENTS. 

Hematoxylin  is  one  of  the  most  generally  useful 
of  the  staining  agents.  It  has  the  power  of  coloring 
certain  parts,  as  the  nuclei  of  cells,  deeply,  while 
other  parts  are  stained  much  less,  or  not  at  all. 
The  following  is  Prof.  Delafield's  method  of  prepar- 
ing the  solution  :  To  make  six  hundred  c.  c.  of  the 
solution,  take  four  hundred  c.  c.  of  saturated  solu- 
tion of  ammonia  alum,  with  an  excess  of  the  alum 
crystals,  and  add  to  this  four  grms.  of  hematoxylin 
(Merck's  is  preferable,  the  crude  extract  will  not 
answer),  dissolved  in  twenty-five  c.  c.  strong  com- 
mercial alcohol.  This,  at  first,  produces  a  light 
violet,  or  sometimes  a  dirty-red  color,  but  on  ex- 
posure to  the  light,  in  an  unstoppered  bottle,  the 
color  deepens;  after  standing  for  three  or  four  days 
exposed  to  the  air  and  light,  the  solution  is  filtered, 
and  one  hundred  c.  c.  each,  of  glycerin  and  Hast- 


INTRODUCTION.  J 

ings'  wood  naphtha  (pyroxylic  spirit)  arc  added. 
The  solution  is  now  allowed  to  stand  for  a  day  or 
two,  and  is  then  filtered  again,  and  this  filtration  is 
repeated,  after  standing,  until  a  dark  sediment  is 
no  longer  formed.  The  color  is  now  usually  very 
deep,  and  the  solution  should  be  kept  in  a  tightly- 
stoppered  bottle. 

Such  a  solution  is  usually  to  be  diluted  with  water 
before  using,  the  exact  degree  of  dilution  depending 
upon  the  rapidity  with  which  we  wish  the  specimen 
to  be  stained.  As  a  rule,  slow  staining  with  a  dilute 
solution  gives  the  best  result,  and  is  less  likely  to 
cause  shrinkage  of  the  specimen.  In  staining,  bits 
of  tissue  are  placed  in  a  small  dish  of  the  solution, 
so  that  they  are  bathed  on  all  sides  by  it,  and  al- 
lowed to  remain  until  sufficiently  colored.  The 
time  required  will  depend  on  the  strength  of  the 
solution,  and  also,  to  a  considerable  degree,  upon 
the  character  and  previous  preparation  of  the  tissue. 
The  excess  of  coloring  fluid  is  to  be  thoroughly 
washed  out  of  the  specimen  by  water  before  further 
manipulation. 

Carmine. — This  is  a  red  stain,  and  is  employed  in 
the  same  manner  as  haematoxylin,  and  is  useful 
where  we  do  not  wish  as  great  body  of  color  in  the 
specimen  as  the  hoematoxyiin  imparts.  It  may  be 
prepared  by  dissolving  two  grms.  of  commercial 
carmine  in  a  few  drops  of  strong  ammonia,  and 
adding  one  hundred  c.  c.  of  water.     It   should   be 


8  NORMAL  HISTOLOGY. 

allowed  to  stand  in  an  open  vessel  until  the  excess 
of  ammonia  evaporates.  As  a  rule,  old  carmine  so- 
lutions stain  better  than  those  recently  prepared. 

Picro-carminate  of  Ammonia,  or  Picro-carminc. — 
With  this  substance  we  can  obtain  at  once  a  yellow 
color  from  the  picric  acid  in  certain  elements,  while 
others  are  stained  red  by  the  carmine.  It  is  pre- 
pared by  adding  to  a  saturated  solution  of  picric 
acid,  a  strong,  ammoniacal  solution  of  carmine  to 
saturation  ;  evaporating  the  mixture  to  one-fifth  its 
bulk;  allowing  to  cool;  filtering  from  the  deposit, 
and  evaporating  the  filtrate  to  dryness  over  a  water 
bath.  The  picro-carmine  is  left  in  the  form  of  an 
ochre-red  powder,  which,  for  use,  may  be  dissolved 
in  water  in  the  proportion  of  one  to  one  hundred. 

Eosin. — This  substance  stains  tissues  more  uni- 
formly than  many  other  dyes,  and  is  especially 
valuable  when  used  in  connection  with  other  color- 
ing agents,  such  as  hematoxylin,  which  stains  the 
cell  nuclei  more  deeply,  since  by  this  method  of 
double  staining  we  have  certain  elements  exhibiting 
one  color,  others  another.  Eosin  may  be  conveni- 
ently used  either  in  aqueous  or  alcoholic  solutions 
of  one  to  one  hundred. 

METHODS    OF    PREPARING    SPECIMENS    FOR    STUDY. 

Certain  fluid  tissues,  such  as  blood,  lymph,  etc., 
are  fitted  for  study,  either  fresh,  or  after  suitable 
preservation,  when  a  drop  is  placed  on  a  slide,  and 


INTRODUCTION.  9 

covered.  Certain  tissues  occur  in  the  form  of  mem- 
branes, of  sufficient  thinness  to  admit  of  study  with- 
out other  manipulation  than  spreading  them  out 
smoothly  on  a  slide.  In  other  cases  we  have  re- 
course to  the  dissociation  of  tissues  by  needles, 
called  teasing. 

Section  Cutting. — In  many  cases  we  wish  to  study 
the  structural  elements  of  a  tissue  in  their  normal 
relations  to  one  another,  and  in  parts  which  are  too 
thick  co  permit  a  direct  observation.  In  such  cases 
we  have  recourse  to  thin  sections,  cut  from  the 
tissue  by  a  sharp  knife  or  razor,  the  tissue,  if  not 
sufficiently  hard  naturally,  being  hardened  by  one 
or  other  of  fhe  above-described  methods.  The  razor 
employed  for  this  purpose  should  have  a  thin  blade, 
perfectly  flat  on  the  lower  side,  and  somewhat  con- 
cave on  the  upper  side,  so  that  a  small  quantity  of 
fluid  will  lie  upon  it.  A  flat,  shallow  dish  is  partly 
filled  with  alcohol,  with  which  the  surface  of  the 
specimen  to  be  cut,  as  well  as  the  razor  blade, 
should  be  constantly  covered,  the  blade  being 
dipped  into  the  alcohol,  and  as  much  taken  up  as 
will  lie  upon  it.  The  bit  of  tissue  being  held  firmly 
in  one  hand,  and  the  razor  firmly  but  lightly  in  the 
other,  the  sections  are  made  by  long,  slow,  diagonal 
sweeps  of  the  razor,  the  blade  being  drawn  from 
heel  to  tip  along  the  tissue,  and  not  crowded  di- 
rectly forward.  Much  practice  is  required  for  mak- 
ing large,  thin,  and  even   sections,  and  the  endeavor 


10  NORMAL  HISTOLOGY. 

at  first  should  be  to  get  thin  and  even  sections,  no 
matter  how  small  they  may  bo.  The  razor  should 
never  be  allowed  to  get  dull,  its  edge  being  fre- 
quently freshened  by  a  few  light  passes  along  a 
leathern  strop. 

INJECTIONS. 

It  is  often  desirable  in  studying  the  distribution 
of  the  blood-  or  lymph-vessels  to  fill  them  by  injec- 
tion with  some  colored  substance  by  means  of 
which  their  ramifications  may  be  readily  recognized. 
One  of  the  most  commonly  employed  injecting 
rials  is  a  solution  of  gelatin  colored  with 
Prussian  blue.  This  may  be  prepared  -as  follows  : 
Dissolve  4  grms.  of  gelatin  in  60  c.  c.  of  water  on  a 
water  bath  ;  divide  the  solution  into  two  portions  ; 
to  one  portion  add  4  c.  c.  of  a  saturated  solution  of 
sulphate  of  iron  (green  vitriol),  stirring  constantly  :"::" 
to  the  other  add  first  8  c.  c.  saturated  sol.  of  fer- 
rocyanide  of  potassium,  and  then  8  c.  c.  saturated 
sol.  of  oxalic  acid  :  these  two  portions  are  now  to 
be  slowly  mixed  with  constant  stirring,  and  then 
heated  up  to  about  the  boiling  point  of  water. 
The  solution  is  now  filtered  hot  through  flannel, 
and  is  ready  for  use.  An  animal  or  organ  injected 
with  this  mixture  should  be  kept  warm  during  the 
injection,  as   should    all    the  utensils  employed,  so 

*  Should  the  iron  cause  a  pasty  precipitate  in  the  gelatin,  this 
portion  should  be  allowed  to  cool,  when  on  warming  again,  with 
stirring,  it  will  dissolve. 


INTRODUCTION.  II 

that  Ihc  gelatin  may  not  harden  prematurely  and 
stop  the  vessels.  The  injection  may  be  made 
with  a  syringe,  or  better  with  some  form  of  appa- 
ratus furnishing  a  constant  pressure  of  variable  de- 
gree. 

EMBEDDING. 

It  often  occurs  that  a  bit  of  tissue  from  which  we 
wish  to  prepare  a  section  is  too  small  or  delicate  to 
be  held  in  the  fingers ;  in  such  cases  the  object  may 
be  placed  between  two  bits  of  hardened  tissue, 
such  as  liver,  tied  round  with  a  thread,  and  thus  held 
while  the  sections  are  made.  Or,  such  a  specimen 
may  be  embedded  in  a  mixture  of  equal  parts  of 
white  wax  and  paraffin  melted  together,  with  ad 
dition  of  a  sufficient  quantity  of  olive  oil  to  giv 
the  mass  the  proper  consistence  for  cutting  whf  i 
cold. 

Certain  tissues  are  very  friable,  so  that  thin  sec- 
tions fall  apart  as  soon  as  they  are  made  ;  or  they 
may  contain  cavities  so  that  they  do  not  afford  suf- 
ficient resistance  to  the  razor.  In  such  cases  the 
interstices  may  be  filled  with  some  fluid  material 
which  can  afterward  be  rendered  solid  and  resis- 
tant, so  that  the  whole  forms  a  firm  mass  ;  after  the 
sections  are  cut  the  impregnating  material  may  be 
dissolved  out,  so  as  not  to  interfere  with  study. 
The  details  of  such  impregnating  processes  will  be 
found  on  p.  1 15. 


12  NORMAL  HISTOLOGY 


MOUNTING. 


Sections,  bits  of  dissociated  tissue,  membranes, 
etc.,  having  been  duly  prepared,  they  are  to  be 
mounted  on  a  slide  for  study.  The  choice  of  a 
a  fluid  for  this  purpose  will  depend  upon  the  nature 
of  the  specimen,  the  mode  of  preparation  to  which 
it  has  been  subjected,  and  the  structural  features 
which  we  wish  especially  to  demonstrate.  One  of 
the  fluids  most  frequently  employed  for  this  pur- 
pose is  glycerin.  Many  of  the  hardening  agents, 
such  as  alcohol,  precipitates,  as  above  remarked, 
certain  albuminoid  substances  in  the  tissues,  in  the 
form  of  minute  strongly  refractive  particles,  thus 
rendering  them  more  or  less  opaque,  or  at  least 
translucent.  Now  glycerin  has  the  power  of  pen- 
etrating many  such  tissues ;  and  since,  as  a  rule,  its 
index  of  refraction  is  much  more  nearly  like  that 
of  the  albuminous  particles,  than  is  the  refractive 
index  of  the  substance  lying  between  them,  when 
the  tissue  becomes  soaked  with  glycerin  the  light 
passes  more  directly  through,  and  the  tissue  is 
more  transparent.  Many  specimens,  furthermore, 
preserve  their  structural  features  very  perfectly  for 
a  long  time  in  glycerin. 

The  stained  specimens  are  either  soaked  until 
they  become  transparent  in  a  small  dish  of  glycer- 
in, and  then  transferred  to  a  slide  and  mounted  in 
the  same  ;  or  they  may  be  mounted  at   once,  with- 


INTRODUCTION.  1 3 

out  the  preliminary  soaking.  Eosin  is  soluble  in 
glycerin,  so  that  if  a  tissue  stained  in  it  is  mounted 
in  pure  glycerin  it  gradually  fades.  To  prevent 
this  the  glycerin  used  for  mounting  should  be 
slightly  tinged  beforehand   with  eosin. 

The  strong  refractive  power  of  glycerin,  although 
of  value  in  rendering  tissues  transparent,  is,  how- 
ever, in  some  cases  prejudicial  to  our  aims,  because 
it  makes  them  too  transparent  ;  its  refractive  power 
being  so  nearly  like  that  of  the  tissue  elements 
themselves  that  their  more  minute  structure  is  con- 
cealed. For  we  find,  within  certain  limits,  that  the 
greater  the  difference  between  the  refractive  power 
of  an  object  and  that  of  the  fluid  in  which  it  lies, 
the  more  distinct  will  be  the  outlines  of  the  object. 

We  have  then  in  using  glycerin  as  a  mounting- 
medium  which  renders  tissues  transparent,  to  guard 
against  its  too  excessive  action  ;  and  this  may  be 
done  by  mixing  with  it,  in  varying  proportions, 
some  less  refractive  substance,  such  as  water.  For 
permanent  preservation,  however,  most  tissues  are 
to  be  put  into  pure  or  neariy  pure  glycerin. 

Canada  balsam  is  for  many  tissues  a  most  excel- 
lent mounting  medium.  It  possesses  to  a  still 
greater  degree  than  glycerin  the  power  of  render- 
ing them  transparent,  obscuring  proportionately,  in 
the  manner  above  described,  certain  of  their  min- 
ute structural  features.  This  difficulty  can,  how- 
ever, be  to  a  considerable  extent  obviated  with  bal- 


14  NORMAL  HISTOLOGY. 

sam  as  with  glycerin,  by  the  judicious  use  of  col- 
oring agents.  Thus,  for  example,  suppose  we  have  an 
object  containing  cells,  the  exact  outlines  of  which 
we  wish  to  bring  clearly  into  view  ;  if  we  stain  with 
hematoxylin  alone,  and  then  mount  the  object  in 
balsam,  the  nuclei  will  be  distinctly  seen,  because 
they  have  a  violet  color  ;  but  the  cell  body  will  in 
many  cases  be  well  nigh  invisible,  because  it  has  al- 
most the  same  refractive  power  as  the  balsam 
which  surrounds  it.  If,  however,  before  mounting 
in  balsam  we  stain  with  eosin,  which  colors  the  cell 
body,  this  will  be  distinctly  visible  under  any  cir- 
cumstances. We  shall  stain  most  specimens  which 
are  to  be  mounted  in  balsam  first  with  haematoxy- 
lin  and  then  with  eosin,  and  throughout  this  man- 
ual, when  the  direction  is  given  to  "stain  a  speci- 
men double,"  this  use  of  haematoxylin  and  eosin  is 
to  be  understood. 

The  most  convenient  wav  of  usin^  Canada  bal- 
sam  is  in  solution  in  chloroform.  The  commercial 
balsam  is  thinned  with  chloroform  until  it  drops 
readily  from  the  end  of  a  glass  rod.  The  mode  of 
procedure  in  Canada  balsam  mounting  is  the  follow- 
ing: The  specimen  having  been  suitably  prepared 
and  stained,  it  is  freed  as  completely  as  possible 
from  water  by  touching  its  edges  with  a  bit  of  fil- 
ter paper,  and  then  placed  in  a  small  wide-mouthed 
bottle  containing  a  few  cubic  centimetres  of  common 
strong  alcohol ;  after  from  ten  to  fifteen  minutes  it  is 


INTRODUCTION.  1 5 

transferred  to  absolute  alcohol,  where  it  remains  fif- 
teen minutes.  It  is  then  taken  out,  the  superfluous  al- 
cohol removed  by  filter  paper,  and  laid  in  oil  of  cloves. 
As  soon  as  it  becomes  transparent,  when  it  usually 
sinks  to  the  bottom  of  the  dish,  it  is  spread  on  a 
slide,  the  excess  of  oil  removed,  a  drop  of  balsam 
put  upon  it.  and  covered  by  thin  glass.  This  pro- 
cess, which  seems  at  first  somewhat  complicated,  is 
readily  understood,  if  we  remember  that  neither 
the  water  in  which  the  specimen  usually  lies  when 
the  staining  is  completed,  nor  the  alcohol,  which,  be- 
ing hygroscopic,  removes  the  water,  are  miscible 
with  balsam  ;  and  further,  that  alcohol  is  micsible 
with  oil  of  cloves,  and  oil  of  cloves  with  balsam. 
Care  must  be  taken  in  these  manipulations  not  to 
breathe  on  the  specimen,  nor  to  allow  any  moisture 
to  come  in  contact  with  it,  since  even  a  small 
amount  of  moisture  produces  a  precipitate  in  the 
balsam  which  greatly  diminishes  the  clearness  of 
the  preparation.  Canada  balsam  is,  as  a  rule,  best 
adapted  for  mounting  those  specimens  in  which  we 
wish  to  study  the  general  structure  of  a  tissue 
rather  than  its  more  minute  characters.  Prepara- 
tions in  which  the  blood  or  lymph  vessels  are  in- 
jected with  some  colored  material,  usually  show 
best  when  mounted  in  balsam. 

To  make  the  double  staining  with  hematoxylin 
and  eosin,  as  mentioned  above,  we  may  first  stain  in- 
the  usual  way  with  an   aqueous   solution  of  haema- 


1 6  NORMAL  HISTOLOGY. 

toxylin,  and  then  accomplish  the  eosin  staining  by 
adding  a  few  drops  of  a  saturated  solution  of  eosin 
in  absolute  alcohol  to  the  absolute  alcohol  used  for 
the  final  dehydration  of  the  specimen,  before  laying 
it  in  oil  of  cloves.  We  thus  complete  the  dehydra- 
tion and  stain  with  eosin  at  one  operation. 

Acetic  acid  is  sometimes  used  to  render  tissues 
transparent.  This  it  does  by  causing  the  albumi- 
noid materials  and  particles  within  them  to  swell  and 
become  actually  more  transparent,  differing  in  this 
from  glycerin  and  balsam.  It  produces,  however, 
very  considerable  changes  in  the  form  and  character 
of  many  tissue  elements,  so  that  although  very  val- 
uable for  special  purposes,  as  will  presently  be  seen, 
it  is  now  much  less  frequently  employed  than  for- 
merly. 

The  chemical  agents  which  the  histologist  uses, 
and  the  manipulative  devices  to  which  he  has  re- 
course in  the  study  of  the  tissues,  are  very  numer- 
ous, and  we  have  considered  here  only  a  few  of  the 
more  important  and  typical.  The  preparation  of 
each  tissue  presents  to  the  worker  in  histology  a 
separate  problem,  and  in  few  departments  of  science 
is  careful  attention  to  technical  minutise  of  more  im- 
portance than  in  that  which  is  now  to  engage  us. 


CHAPTER  I. 
THE   CELL   IN  GENERAL. 

In  all  animal  bodies  arc  found  certain  tiny  struc- 
tural elements  called  cells.  These  are  parts  which 
at  one  time  or  another  are  alive  ;  and  all  the  varied 
activities  of  the  body  are  the  result  of  the  single  or 
combined  activities  of  the  cells  which  are  within  it. 
It  is  very  desirable,  owing  to  their  great  significance, 
that  before  commencing  the  systematic  study  of  the 
tissues  we  should  acquire  a  definite  conception  of 
the  nature  of  these  elementary  organisms.  We  may 
consider  cells  from  a  morphological  and  from  a 
physiological  standpoint,  asking  first,  what  is  their 
structure  ?  and  second,  what  do  they  do  ? 

First,  then,  what  is  the  structure  of  cells  ?  We 
find  in  this  a  great  diversity,  some  being  extremely 
simple,  others  quite  complex.  The  most  common, 
and  usually  the  most  prominent  structural  feature 
of  the  cell,  that  portion  which  gives  to  it  form  and 
consistence,  is  called  the  cell-body.  This  usually  con- 
sists of  an  albuminoid  material,  sometimes  transpar- 
ent and  apparently  structureless,  sometimes  finely 
or  coarsely  granular,  and   not   infrequently  present- 

17 


1 8  NORMAL  HISTOLOGY. 

ing,  at  least  after  death,  a  reticulated  appearance ; 
this  material  is  called  protoplasm  in  its  typical  active 
form,  and  maybe  then  very  soft  and  viscid,  like  thin 
jelly,  or  it  may  become  so  modified  as  to  be  hard 
and  even  horny  like  the  nail.  The  cell-body  pre- 
sents a  great  variety  of  forms  ;  it  may  be  spherical, 
cuboidal,  cylindrical,  fusiform,  ovoid,  pear-shaped, 
discoidal,  or  scale-like  ;  it  often  sends  off  processes 
like  branches  or  wings,  and  sometimes  assumes  the 
most  irregular  bizarre  forms.  We  not  infrequently 
find  imbedded  in  the  cell-body,  pigment  granules, 
droplets  of  fat,  and  various  kinds  of  crystals.  The 
form  of  the  cell-body  seems  usually  to  depend  large- 
ly upon  the'  pressure  to  which  it  is  or  has  been 
subjected  by  adjacent  structures. 

Within  the  cell-body  and  making  up  a  part  of  the 
protoplasm,  we  usually  find  one  or  more  spherical, 
ovoidal,  or  irregular-shaped  bodies,  called  nuclei 
(singular,  nucleus).  The  nucleus  usually  presents  a 
sharply  defined  contour,  is  frequently  coarsely  gran- 
ular, and  sometimes  a  coarse  or  filiform  network  is 
seen  suspended  within  or  stretching  from  side  to 
side.  The  nucleus  may  be  very  small  in  proportion 
to  the  size  of  the  cell-body,  or  may  make  up  nearly 
the  entire  bulk  of  the  cell.  In  the  processes  of  de- 
generation and  decomposition,  and  under  the  action 
of  certain  chemical  agents,  the  nucleus  is  more  re- 
sistent  than  the  cell-body,  and  on  treatment  of  the 
cell  with  certain  coloring  agents,  such  as  hsematoxy- 


THE  CELL  IN  GENERAL.  1 9 

lin,  the  nucleus  is  more  deeply  stained  than  the  cell- 
bod}*.  Within  the  nucleus,  again,  we  frequently 
find  one  or  more  small  spherical  or  irregular-shaped 
bodies,  looking  like  vesicles  or  shining  granules, 
which  are  called  nucleoli.  They  would  seem,  in 
some  cases  at  least,  to  be  connected  with  the  above- 
mentioned  intra-nuclear  network.  Of  the  exact 
nature  and  significance  of  the  nucleus  and  nucleolus, 
we  have  at  present  little  definite  knowledge,  but  the 
nucleus,  at  least,  seems  to  be  a  very  important  part 
of  the  cell. 

Finally,  a  small  proportion  of  animal  cells  are 
enclosed  by  an  envelope — the  celt-membrane \  which 
may  be  thick  or  thin,  now  presenting  well-defined 
structural  peculiarities,  and  again  quite  homo- 
geneous and  structureless.  In  many  cases  the  cell- 
membrane  would  seem  to  be  simply  a  peripheral 
hardened  layer  of  the  cell-body. 

It  will  thus  be  seen  that  we  may  have,  in  an  animal 
cell,  four  distinct  structural  elements  :  the  body,  the 
nucleus,  the  nucleolus,  and  the  membrane.  It  is  only 
in  a  few  varieties  of  cells,  however,  that  all  of  these 
elements  are  present.  The  cell-membrane  is  the 
least  commonly  present  of  all,  and  there  are  certain 
cells  which  consist  of  a  cell-body  alone. 

Regarding  cells  now  from  a  physiological  point  of 
view,  we  find  the  expression  of  their  vitality  in  four 
distinct  ways :  they  nourish  themselves  ;  they  grow ; 
they  perform  certain  functions  for  their  own  good  and 


20  NORMAL  HISTOLOGY. 

for  the  benefit  of  the  organism  of  which  they  form  a 
part;  and,  finally,  under  certain  circumstances,  they 
are  capable  of  reproducing  their  like.  Or,  in  more 
concise  language,  we  say  the  cell  expresses  its 
vitality  in  nutrition,  growth,  function,  and  reproduc- 
tion. Not  all  of  these  expressions  of  vitality,  how- 
ever, can  be  subjected  to  direct  microscopical 
observation.  Nutrition,  being  as  we  believe,  essen- 
tially a  chemical  process,  cannot,  with  our  present 
facilities,  become  to  any  considerable  extent  the  ob- 
ject of  direct  microscopical  study.  The  growth  of 
cells,  too,  is  for  the  most  part  so  gradual,  that  we 
cannot  directly  follow  it,  but  are  obliged  to  study  it 
in  different  phases  of  its  progress. 

The  functional  activity  of  cells  can  be  indirectly 
subjected  to  microscopical  investigation  when  it  is 
associated  with  demonstrable  changes  in  the  mor- 
phological characters  of  the  cell,  or  directly  observed 
when  it  expresses  itself  in  motion  ;  thus,  we  can 
readily  detect  the  difference  between  a  condition  of 
functional  activity  and  rest  in  the  peptic  cells  of 
the  stomach,  and  the  observation  of  ciliary  and 
amoeboid  movements  in  certain  cells  is  among  the 
most  fascinating  of  histological  studies. 

Finally,  regarding  the  reproduction  of  cells,  in  a 
few  instances  the  act  has  been  directly  observed 
under  the  microscope,  but  in  the  majority  of  cases 
our  knowledge  is  derived  from  the  study  of  a  suc- 
cession of  consecutive  stages  in  the  process.     Every 


THE  CELL  IN  GENERAL.  2  1 

new  cell  which  appears  in  the  animal  body,  during 
and  subsequently  to  its  development,  is  derived 
from  a  pre-existing  cell,  and  all  the  cells  are  deriva- 
tives of  a  single  original  cell,  the  ovum.  New  cells 
are  produced  by  a  process  of  division  in  older  cells. 
Cell-division  seems  to  occur  in  a  variety  of  ways, 
usually  commencing  in  the  nucleus.  Of  the  details 
of  the  process  we  have  at  present  but  little  satisfac- 
tory classified  knowledge,  and  although  interesting 
and  valuable  facts  are  rapidly  accumulating,  we  can- 
not tarry  long  upon  the  subject  here. 

In  one  of  the  apparently  simplest  modes  of  cell- 
division,  one  or  more  constrictions  appear  around 
the  cell-body,  after  certain  changes  in  the  nucleus* 
have  led  to  its  partial  or  entire  division  ;  these  con- 
strictions grow  deeper  and  deeper,  until  finally  the 
parts  separate  and  become  individual  cells,  each  fur- 
nished with  a  nucleus.  This  form  of  cell-division  is 
called  reproduction  by  fission.  Or,  the  cell-body 
may  send  off  from  some  part  a  bud-like  process, 
which,  becoming  nucleated,  is,  by  a  constriction  of 
its  pedicle,  at  length  set  free  as  an  independent 
cell ;  this  is   called   cell-reproduction  by  budding  or 

*  These  preliminary  changes  in  the  nuclei  of  proliferating  cells, 
which  consist  in  a  readjustment  and  perhaps  new  formation  of  fibres 
of  the  intra-nuclear  network,  and  the  development  of  bizarre  and 
often  symmetrical  figures  -within  the  nucleus,  are  at  present  engaging 
the  attention  of  many  eminent  histologists.  The  results  of  their  re- 
searches, while  now  affording  much,  and  promising  far  deeper,  insight 
into  the  nature  of  cell-life,  are  not  yet  sufficiently  classified,  and  are 
far  too  intricate  to  come  within  the  scope  of  this  manual. 


22  NORMAL  HISTOLOGY. 

gemmation.  A  mode  of  division,  called  endogenous 
cell-reproduction,  is  described,  in  which  the  division 
is  said  to  occur  entirely  within  the  membrane  of  a 
cell,  called  the  parent  cell,  so  that  the  new  organ- 
isms— the  daughter  cells — are  not  at  once  set  free. 
This  mode  of  cell-division  is,  however,  doubtful,  or 
at  least  extremely  infrequent.  These  modes  of  cell- 
reproduction  seem  to  be  different,  and  yet  there  is 
much  reason  for  believing  that  they  are  really  only 
modifications  of  one  process  ;  but  in  the  present 
condition  of  our  knowledge  on  the  subject,  all 
general  statements  should  be  very  cautiously  made  ; 
and  such  classifications  as  the  above  are  to  be  re- 
garded merely  as  for  convenience  of  study,  and  not 
as  expressing  any  absolute  and  fully  established 
truth. 

Cells  are  variously  classified  according  to  their 
nature  and  relations  to  adjacent  parts  :  thus  we  have 
epithelial  cells,  which  cover  the  skin  and  mucous 
membranes,  and  occur  in  certain  parts  of  the  glan- 
dular organs ;  connective-tissue  cells,  which  lie  scat- 
tered throughout  the  substance  of  the  structures, 
presently  to  be  described  as  connective  tissue,  and 
in  certain  parts  undergoing  modification  of  form 
and  relation  to  neighboring  parts,  and  called  endo- 
thelial cells ;  gland  cells  are  those  which,  possessing 
peculiar  functional  or  morphological  characters, 
make  up  the  parenchyma  of  certain  glands  and  or- 
gans.     The   special   characters   of    these  classes  of 


THE  CELL  IN  GENERAL.  2  3 

cells,  together  with  those  of  other  classes  not  here 
mentioned,  will  be  considered  in  our  systematic 
study  of  the  tissues. 

PRACTICAL  STUDY. 

Many  of  the  above-described  general  characters  of 
cells  may  be  seen  by  studying  the  epithelial  cells  of  the 
bladder,  the  pigmented  cells  of  the  retina,  and  the  living 
ciliated  cells  from  the  mucous  membrane  of  the  frog's 
mouth. 

Epithelial  Cells  from  the  Rabbit 's  Bladder. — A  small  piece, 
including  the  mucous  membrane,  is  cut  from  the  wall  of 
the  bladder  of  a  recently-killed  rabbit  and  immersed  for 
24  hours  in  a  5-per-cent.  solution  of  neutral  chromate  of 
ammonium.  It  is  then  soaked  for  an  hour  in  water  to 
remove  the  chromate  solution,  and  the  now  loosened  cells 
are  scraped  from  the  surface  of  the  mucous  membrane 
and  placed  in  a  few  cubic  centimetres  of  the  picro-car- 
mine  solution,  where  they  remain  until  they  have  acquired 
a  distinct  red  color.  A  small  fragment  of  the  cell  mass  is 
now  transferred  ta  a  slide,  covered  with  a  drop  of  gly- 
cerin, teased  apart  with  needles,  and  carefully  covered 
with  thin  glass  so  as  to  exclude  all  bubbles  of  air.  The 
cover-glass  should  be  allowed  to  close  down  upon  the 
specimen  by  its  own  weight  alone,  since  much  pressure 
upon  these  delicate  cells  distorts  and  breaks  them.  This, 
like  nearly  all  specimens,  should  be  examined  first  with  a 
low  and  then  with  a  high  power.  The  cells  will  be  found 
to  have  a  great  variety  of  forms,  some  of  them  evidently 
determined  by  the  pressure  of  adjacent  cells.    They  have 


24  NORMAL  HISTOLOGY. 

a  finely  granular  body,  with  more  coarsely  granular  nuclei, 
and  nucleoli.  In  many  cases,  instead  of  appearing 
coarsely  granular,  the  nucleus  is  seen  to  contain  a  distinct 
reticulum  {intra-nuclear  net-work)  of  strongly  refractive 
material,  the  thickened  nodal  points  of  which  may  be  re- 
garded as  nucleoli.  To  preserve  this,  and  all  specimens 
mounted  in  glycerin,  a  narrow  rim  of  asphalt  varnish  is 
painted  around  the  cover-glass,  lapping  over  the  edges  of 
the  latter  and  extending  for  a  short  distance  on  to  the 
slide. 

Pigmented  Cells  of  the  Retina. — An  eye  from  the  ox  or 
sheep,  which  has  lain  for  a  few  days  in  Miiller's  fluid,  is 
opened  by  an  equatorial  section,  and  the  vitreous  body 
and  the  retina  removed  from  the  posterior  half.  If  the 
remaining  portion,  including  the  sclera,  choroid,  and  a 
portion  of  the  pigmented  epithelium  of  the  retina,  be  put 
under  water  in  a  shallow  dish,  ana  brushed  gently  over 
the  surface  with  a  fine  camel's-hair  pencil,  delicate  pig- 
mented flakes  will  float  off  in  the  water.  These  are  the 
desired  cells.  One  or  two  bits  should  be  put  into  a  drop 
of  glycerin  on  a  slide.  Another  bit  is  put  on  to  another 
slide  with  a  large  drop  of  haematoxylin  solution,  and  after 
about  ten  minutes  the  coloring  fluid  carefully  washed  off 
with  water,  and  the  stained  fragments  placed  with  the 
other  in  the  glycerin  on  the  other  side.  They  are  now 
covered  and  examined. 

These  cells  appear  hexagonal*   and  are  joined  together 

*  These  cells  have  really  a  very  complicated  structure,  see  page 241, 
but  it  is  not  necessary  to  study  this  in  detail  here,  since  they  are  only 
studied  now  for  the  purpose  of  demonstrating  the  occurrence  of  pig- 
ment in  the  bodies  of  certain  cells. 


TJIE  CELL  IX  GENERAL.  2$ 

edge  to  edge,  giving  a  pavemented  appearance  to  the  frag- 
ments. Most  of  the  cell  bodies  are  closely  crowded  with 
elongated  brown  or  black  pigment  granules.  Sometimes 
such  are  seen  as  have  but  few  granules  within  them,  and 
occasionally  they  are  entirely  free.  When  the  cell  bodies 
are  crowded  with  pigment  the  nucleus  appear  as  a  rounded, 
indefinitely  outlined  structure,  containing  no  pigment  and 
looking  like  a  hole  'in  the  cell.  When  the  pigment  is 
present  in  small  quantity  or  entirely  absent,  the  nucleus 
is  much  less  sharply  outlined.  In  the  cells  which  have 
been  stained  with  hematoxylin,  however,  the  nuclei  all 
present  well-defined  outlines,  and  are  stained  of  a  violet 
color. 

Ciliary  Movement  in  Living  Ciliated  Cells  from  the  Frogs 
Mouth. — The  mucous  membrane  of  the  roof  of  the  mouth 
of  a  living  frog  is  gently  scraped  with  a  scalpel,  and  the 
slimy  mass  thus  procured  is  transferred  to  a  large  drop 
of  one-half-per-cent.  salt  solution  on  a  slide,  and  thor- 
oughly teased  apart.  The  specimen  is  now  covered,  a  bit 
of  hair  being  placed  beforehand  beside  it  to  prevent  pres- 
sure from  the  cover-glass.  The  cells  are  mostly  sphe- 
roidal, and  will  be  seen  isolated  or  in  clusters,  the  cilia, 
in  the  form  of  rows  of  delicate  hair-like  processes,  spring- 
ing from  one  side  of  the  cells.  A  considerable  number 
of  non-ciliated  cells  will  also  be  observed.  The  form  of 
the  cilia  will  be  most  readily  seen  in  cells  in  which  the 
movement  is  becoming  languid,  which  occurs  gradually  in 
all,  and  finally  ceases  altogether.  The  movement,  when 
vigorous,  often  causes  cells  and  masses  of  cells  to  revolve 
and  move  about  in  the  fluid,  and  frequently  produces 


26  NORMAL  HISTOLOGY. 

currents  in  the  latter,  into  which  floating  particles  are 
drawn  and  then  driven  onward  with  considerable  velocity. 
If  it  be  desired  to  preserve  specimens  of  ciliated  cells, 
a  fragment  of  the  palate  of  the  frog,  or,  better,  of  the 
trachea  of  the  rabbit — since  the  cells  here  are  much  more 
like  those  in  the  air-passages  of  man, — should  be  excised 
and  treated  in  the  manner  above  described  for  the  prepa- 
ration of  the  epithelial  cells  of  the  bladder. 


CHAPTER  II. 
CONNECTIVE   TISSUE. 

OUR  knowledge  of  the  animal  tissues  is  not  yet 
extensive  and  exact  enough  to  enable  us  to  make  a 
satisfactory  classification  of  them,  but  for  conven- 
ience of  study  we  may  regard  the  body  as  composed 
of  simple  tissues  and  of  organs  As  examples  of 
the  first  we  have  connective  tissue,  muscular  tissue, 
nerve-tissue,  etc.  ;  of  the  second,  the  liver,  the 
lungs,  the  skin,  mucous  membranes,  etc. 

Among  the  simple  tissues  there  is  a  large  and  im- 
portant group,  called  connective  tissues,  the  members 
of  which,  though  presenting  many  marked  differ- 
ences, yet  seem  so  closely  allied,  both  in  structure 
and  life  history,  as  to  justify  their  grouping  under 
the  above  common  name.  The  members  of  this 
group  of  tissues  may  be  tabulated  as  follows:* 

I.     Fibrillar  connective  tissue. 

*  In  addition  to  these  varieties,  we  find  in  certain  parts  of  the  body 
membranous  layers  or  sheaths — sometimes  structureless,  sometimes 
having  well-marked  structural  features— which  differ  in  many  re- 
spects from  the  above-mentioned  varieties  of  connective  tissue,  but 
which  cannot  be  separately  described  here.  They  will  be  briefly 
considered  as  we  meet  with  them  in  our  systematic  study  of  the  parts 
of  the  body  in  which  they  occur. 

27 


28  NORMAL  HISTOLOGY. 

2.  Embryonal  and  mucous  tissue. 

3.  Fat  tissue. 

4.  Reticular  connective  tissue. 

5.  Cartilage. 

6.  Bone  and  teeth. 

Aside  from  many  striking  points  of  similarity  in 
their  structure,  which  we  can  better  appreciate  after 
having  made  a  practical  study  of  the  group,  three 
considerations  may  be  briefly  noticed  here  as  of 
weight  in  determining  this  classification.  At  an 
early  period  of  embryonic  life,  when  the  animal  is 
composed  almost  exclusively  of  cells,  it  is  found 
that  one  of  the  first  definite  arrangements  or  group- 
ings of  these  cells  consists  in  the  formation  of  three 
distinct  layers :  an  outer,  a  middle,  and  an  inner 
layer.  Now  it  is  found  that  the  cells  in  these  dif- 
ferent layers  have  an  entirely  distinct  capacity  in 
their  power  of  producing  the  tissues  which  are  found 
in  the  adult  animal.  From  the  cells  in  the  outer 
layer  are  produced  the  skin  with  its  adnexa,  certain 
organs  of  sense,  and  the  central  nervous  system  ; 
from  the  inner  layer  are  formed  the  intestinal  epi- 
thelium and  glands,  and  certain  of  the  large  organs 
of  the  body ;  while  from  the  middle  layer  are  de- 
veloped the  muscles,  the  vascular  systems,  and, 
which  especially  concerns  us  here,  the  tissues  tabu- 
lated above  as  members  of  the  connective-tissue 
group.  Besides  the  relationship  given  to  them  by 
this  common   origin,  these  tissues  show  their  close 


CONNECTIVE   TJSSl  E. 

alliance  by  the  fact  that  during  the  process  of  devel- 
opment one  is  sometimes  formed  from  another. 
Finally,  certain  frequently  observed  pathological 
conditions  seem  to  consist  chiefly  in  the  transforma- 
tion of  one  of  these  forms  of  tissue  into  another  of 
the  same  group. 

With  fibrillar  connective  tissue  or  connective  tissue 
proper,  or  simply  connective  tissue,  as  it  is  often 
called,  we  commence  our  systematic  study.  This 
tissue  is  most  widely  distributed  in  the  human  body, 
occurring  in  the  greatest  diversity  of  forms,  and  an 
exact  knowledge  of  its  structure  and  arrangement  is 
absolutely  essential  to  a  correct  understanding  of 
most  other  tissues  and  the  organs,  since  it  occurs  in 
one  form  or  another  in  nearly  all  of  them.  Fibrillar 
connective  tissue,  under  the  name  of  tendons  and 
ligaments,  forms  bands  and  cords  which  bind  the 
muscles  to  the  bones,  and  bind  the  bones  together. 
It  is  spread  out  in  thin  layers  in  the  fasciae  and 
aponeuroses  ;  it  surrounds  the  bones  and  cartilage 
as  periosteum  and  perichondrium.  It  divides  and 
encloses  muscular  bundles  and  the  nerves  ;  it  sup- 
ports the  blood  and  lymphatic  vessels,  and  forms 
the  limiting  membrane  of  the  serous  cavities.  It 
forms  an  encasing  membrane  for  many  organs,  and, 
extending  into  their  interior,  serves,  under  the 
name  of  interstitial  tissue,  to  support  their  paren- 
chyma. 

Fibrillar  connective  tissue  is  composed  of  two  dis- 


30  NORMAL  HISTOLOGY. 

tinct  classes  of  structures  :  a.  cells,  and  b.  a  substance 
lying  between  the  cells,  the  intercellular  or  basement 
substance.     We  shall  consider  the  latter,  first. 

b. INTERCELLULAR    SUBSTANCE. 

The  intercellular  substance  consists  chiefly  of  two 
distinct  kinds  of  fibres  :  fibrillated  fibres  and  elastic 
fibres. 

The  fibrillated  fibres  are  tiny,  grayish,  translucent 
cords,  which,  sometimes  singly,  sometimes  in  straight 
or  wavy  bundles,  lie  nearly  parallel  to  one  another, 
and  again  cross  each  other  at  all  conceivable  angles, 
forming  complicated  net-works.  When  examined 
fresh,  with  high-magnifying  powers,  the  individual 
fibres  are  seen  to  be  moderately  refractive  ;  they 
have  a  delicate  longitudinal  striation — this  striation 
being,  as  we  shall  presently  see,  the  expression  of 
the  fact  that  each  fibre  is  made  up  of  a  number  of 
still  finer  fibrillar. 

On  boiling  for  a  considerable  time  in  water  they 
are  converted  into  gelatin,  and  when  treated  with 
acetic  acid  or  dilute  alkalies,  they  swell  up,  lose 
their  longitudinal  striations,  become  very  trans- 
parent, and  finally  almost  invisible. 

The  second  variety  of  fibres  which  occurs  in  con- 
nective tissue — the  elastic  fibres — are  much  more 
strongly  refractive  than  the  first,  hence  presenting 
more  sharply-marked  contours  ;  they  are  not  longi- 
tudinallv  striated,  and  often  branch  and  form   anas- 

J 


CONNECTIVE  TISSUE.  3  I 

tomoses  with  one  another,  sometimes  joining  at 
frequent  intervals  to  form  a  narrow-meshed  net,  and 
again  stretching  away  for  long  distances  to  form  a 
broadly-spaced  reticulum.  Sometimes  the  fibres 
are  broad  and  band-like,  sometimes  extremely  fine  ; 
on  being  boiled  in  water  they  are  not  converted 
into  gelatin,  and  they  are  unchanged  by  acetic  acid 
and  dilute  alkalies.  These  fibres,  as  their  name  in- 
dicates, possess  elasticity,  and  as  a  consequence  of 
this  property  we  often  find,  when  the  fibres  have 
been  severed  by  teasing  or  other  modes  of  prepara- 
tion, that  the  free  ends  curl  over  in  the  act  of  re- 
traction, forming  very  characteristic  curves  or 
spirals.  The  elastic  substance  sometimes  occurs  in 
the  form  of  granules  instead  of  fibres,  or  as  nearly 
homogeneous  membranes. 

The  relative  number  of  the  fibrillar  and  elastic 
fibres  in  connective  tissue  varies  greatly  in  different 
parts  of  the  body  :  in  some  we  find  but  few  elastic 
fibres,  others  contain  little  else.  The  interstices  of 
the  interlacing  fibres — both  fibrillated  and  elastic — ■ 
are  filled  with  the  nutritive  fluids  of  the  body  :  or 
in  some  cases,  a  small  amount  of  a  more  consistent 
homogeneous  material  cements  them  together. 
The  marked  difference  in  general  appearance 
which  is  seen  in  different  parts  composed  of  con- 
nective tissue,  is  largely  due  to  differences  in  the 
arrangement  of  the  fibres  and  bundles,  and  in  the 
relative  proportion  of  the  fibrillated  and  elastic 
fibres. 


32  NORMAL  HISTOLOGY. 

a. — CELLS. 

We  consider  next  the  cellular  elements  of  fibril- 
lar connective  tissue.  These  are  of  two  distinct 
classes.  First,  those  which  are  essential  components 
of  it,  preserving  a  fixed  and  definite  relation  to  the 
basement  substance,  and  which  are  quite  constant 
in  the  different  varieties  of  tissue,  in  form,  size, 
and  number  ;  these  are  called  fixed  connective-tissue 
cells.  Second,  small  spheroidal  cells,  the  white 
blood-cells,  which,  escaping  in  varying  number 
from  the  blood-vessels,  move  about  through  the  in- 
terstices of  the  tissues,  and  are  called  wandering 
cells.     These  will  be  studied  with  the  blood. 

Among  the  fixed  connective-tissue  cells,  by  far 
the  greater  proportion  are  more  or  less  flattened, 
presenting  to  the  eye,  when  seen  from  the  edge  or 
in  cross-section,  the  appearance  of  slender  spine. 
The  cell-bodies  are  for  the  most  part  quite  trans- 
parent, often  very  thin  and  scale-like,  frequently  so 
delicate  as  to  be  difficult  of  recognition,  and  some- 
times the  protoplasm  in  the  vicinity  of  the  nucleus 
is  distinctly  granular.  These  fixed  connective- 
tissue  cells  present  a  great  variety  of  forms,  being 
round,  ovoid,  oblong,  fusiform,  or  irregularly  rec- 
tangular, ofcen  sending  off  branches  which  seem 
to  be  connected  with  the  branches  of  neighboring 
ceils.  Sometimes  the  thin  cell-body  sends  off  one 
or  more  delicate  wing-like  processes  at*  varying 
angles.     These  cells  may  have   one  or  more  nuclei  ; 


CONNECTIVE   TISSUE.  33 

they  lie  in  the  interstices  of  the  fibres,  which  they 
often  enwrap  with  their  delicate  bodies.  The  form 
which  they  assume  in  the  different  varieties  of  con- 
nective tissue  would  seem  to  be  largely  dependent 
upon  the  varying  conditions  of  pressure  to  which 
they  are  subjected  by  the  adjacent  fibres.  These 
flattened  connective-tissue  cells  in  many  parts  of 
the  body  may,  and  in  certain  parts  constantly  do, 
contain  pigment  granules. 

In  certain  parts  of  the  body  the  connective  tissue 
presents  free  surfaces,  lining  more  or  less  well-de- 
fined closed  spaces  or  cavities,  or  free  surfaces 
which  are  movable  over  one  another,  as  in  the  great 
serous  cavities,  in  the  blood-  and  lymph-channels, 
tendon-sheaths,  etc.  In  these  cases  the  flat  con- 
nective-tissue cells  usually  undergo  some  modifica- 
tion in  their  form,  character,  and  relations  to  one 
another,  and  are  called  endothelial  cells  or  endothe- 
lium. 

Finally,  in  certain  parts  of  the  body,  usually  in 
the  vicinity  of  blood-vessels,  are  found  irregular- 
shaped  granular  cells,  not  usually  fiat,  and  of  vary- 
ing size  and  form,  called  plasma  cells,  which  resem- 
ble in  many  respects  cells  found  in  the  embryo. 

PRACTICAL   STUDY. 
b. — INTERCELLULAR    SUBSTANCE. 

Fibres  of  Subcutaneous  Connective  Tissue. — To  study 
these,  the  skin  should  be  reflected  back  from  the  abdorai- 


34  NORMAL  HISTOLOGY. 

nal  wall  of  a  recently  killed  animal  (mammal),  and  choos- 
ing a  part  which  is  free  from  fat,  a  bit  of  the  loose,  so-, 
called  areolar  tissue  is  seized  with  the  forceps  and 
snipped  off  with  scissors.  The  bit  of  tissue,  which  will 
contract  to  a  little  lump  around  the  point  of  the  forceps, 
is  to  be  spread  out  very  thin  on  a  slide  and  covered  with 
a  J-per-cent.  salt  solution.  *The  specimen  will  be  seen  to 
consist  largely  of  fibrillated  fibres  crossing  one  another 
in  all  directions,  with  a  few  delicate  elastic  fibres.  After 
studying  in  salt  solution,  a  drop  of  2-per-cent.  sol.  of 
acetic  acid  should  be  allowed  to  run  under  one  edge  of 
the  cover- glass,  the  salt  solution  being  drawn  off  by  a  bit 
of  filter  paper  placed  at  the  opposite  edge,  and  the  effect 
carefully  observed.  This  preparation  is  not  to  be  pre- 
served. 

Fibrillce  in  Tail  Tendon  of  Mouse. — It  is  not  easy  in 
studying  fresh  tissues  to  convince  one's  self  that  the  longi- 
tudinal striations  on  the  fibres  are  really  the  expression 
of  their  fibrillar  structure,  since  any  attempt  to  pick  the 
fibres  apart  with  needles  is  of  little  avail,  because  the 
fibrillse  are  bound  together  by  a  small  amount  of  cement 
substance.  If,  however,  the  tissue  be  placed  for  a  few 
hours  in  some  fluid  which  dissolves  this  cement  substance, 
as  osmic  acid,  the  ultimate  elements,  the  fibrillar,  may  be 
readily  separated.  A  bit  of  the  tail  tendon  of  a  mouse, 
or  a  small  tendon  from  any  mammal,  should  be  soaked 
for  a  few  days  in  a  i-per-cent.  sol.  of  osmic  acid,  washed 
and  teased  apart  on  a  slide  and  mounted  in  glycerin. 

Elastic  Fibres  from  the  Ligamentum  Nuchce. — A  small 
fragment  of  this  structure — conveniently  obtained  from 
ihc  ox — i>    preserved    in    strong  alcohol.     A   tiny  bit    is 


CONNECTIVE  TISSUE.  35 

teased  thoroughly  and  mounted  in  glycerin.  It  will  be 
seen  to  consist  of  a  dense  net-work  of  broad,  closely-an- 
astamosing  elastic  fibres  which  curl  over  at  the  free 
ends. 

a. — CELLS. 

Cells  in  the  Subcutaneous  Connective  Tissue. — The 
fixed  connective-tissue  cells,  which  occur  in  this  form 
of  tissue,  may  be  studied  in  any  mammal  They 
vary  somewhat  in  size,  shape,  and  number  in  differ- 
ent animals,  but  those  in  the  rabbit  are  sufficiently 
typical.  In  the  study  of  these  cells,  whose  bodies  are, 
for  the  most  part,  so  thin  and  transparent  as  to  be 
almost  invisible  when  fresh,  and  very  liable  to  shrink  and 
become  distorted  by  contact  with  the  usual  hardening 
agents,  we  have  to  fulfil  two  important  indications  in  our 
technical  procedure  ;  we  must  treat  the  tissue  with  some 
agent  which  will  render  the  cells  visible,  and,  at  the  same 
time,  not  greatly  alter  the  form  of  the  delicate  cell- 
body. 

These  ends  are  best  attained  by  the  method  suggested 
by  Dr.  W.  H.  Welch.  The  skin  being  reflected  back 
from  the  abdominal  wall  of  a  freshly-killed  rabbit,  with 
a  hypodermic  syringe,  inject  beneath  the  loose  adherent 
tissue  a  few  cubic  centimetres  of  a  solution  of  analine 
red  (Fuchsine),  prepared  by  adding  one  part  of  saturated 
alcoholic  sol.  of  the  powder  to  forty  parts  of  water.  After 
the  red  fluid  has  remained  for  two  or  three  hours  in  con- 
tact with  the  tissues,  a  small  fragment  is  snipped  off  from 
the  colored  cedematous  portion,  spread  very  thin  on  a  slide, 
covered  with   the   same  fluid,  and  enclosed  at  once  With 


36  NORMAL  HISTOLOGY. 

asphalt  varnish.  The  cells  in  this  tissue  are,  for  the 
most  part,  extremely  thin,  and  of  various,  often  quite 
irregular  forms,  sometimes  sending  off  narrow  branching 
processes,  by  which  they  join  neighboring  cells,  and 
sometimes  furnished  with  wing-like  projections.  In  ad- 
dition to  these  cells,  and  the  intercellular  fibres,  nerves 
and  capillary  blood-vessels  are  sometimes  seen  in  the 
specimens,  and,  occasionally,  in  the  vicinity  of  the  ves- 
sels are  found  the  above-mentioned  plasma  cells.  The 
cells  prepared  by  this  method  preserve  their  form  for  a 
considerable  time,  but  finally  shrink,  and  lose  their 
color. 

Pigmented  Connective-tissue  Cells  of  the  Choroid. — These 
may  be  taken  from  the  eye  of  any  mammal  (except  al- 
binos) which  has  been  hardened  in  Miiller's  fluid  and 
alcohol.  A  shred  of  the  outer  layers  of  the  choroid 
should  be  torn  off,  stained  with  hematoxylin,  and 
mounted  in  glycerin.  Irregular- shaped,  often  branched 
.and  flattened  cells  are  seen  lying  embedded  in  a  mem- 
branous nucleated  basement  substance,  containing  deli- 
cate elastic  fibrils,  the  cells  being  more  or  less  crowded, 
except  in  the  part  occupied  by  the  nucleus,  with  a  multi- 
tude of  minute  brown  or  black  granules. 

Cells  in  Teased  Tendon. — Tendons  are  composed  essen- 
tially of  parallel  bundles  of  fibrillated  connective-tissue 
fibres,  with  thin  irregular-shaped  cells  between  them. 
The  complicated  forms  which  tendon  cells  present  can 
be  most  easily  understood  when  they  are  studied  in  their 
natural  relation  to  the  fibres. 

Owing  to  their  extreme  tenuity,  and  the  consequent 
possibility  of  studying  them  with   but  little  manipulation. 


CONNECTIVE   J'JSSUE.  37 

the  tail  tendons  of  the  mouse  or  rat  are  well  suited  for 
this  purpose.  If  the  skin  of  the  tail  of  a  recently-killed 
mouse  or  rat  be  torn  across  transversely,  by  slight  twist- 
ing, or  pressure  with  the  edge  of  the  nail,  and  gentle 
traction  be  exerted  at  both  sides  of  the  rupture,  the 
caudal  vertebrae  will  readily  separate,  and,  as  the  trac- 
tion continues,  a  multitude  of  extremely  fine  tendon- 
bundles  will  be  seen  stretching  across  the  break.  When 
the  tendons,  by  this  natural  and  simple  dissection,  have 
been  freed  for  one  or  two  centimetres  of  their  length, 
they  are  laid,  still  under  slight  tension,  on  a  bit  of  glass, 
of  suitable  size,  the  ends  cemented  to  the  glass  with  a  bit 
of  soft  wax  or  paraffin,  and  the  remainder  of  the  tail  cut 
off.  The  fastening  of  the  ends  is  to  prevent  the  tendon 
from  shrinking  when  immersed  in  the  preservative  fluid. 
The  bit  of  glass,  with  its  tendon-bundles,  is  now  immersed 
for  twenty-four  hours  in  a  one-per-cent.  solution  of  osmic 
acid,  then  washed  in  water. 

One  or  two  of  the  tiny  bundles  is  now  to  be  partially 
teased  apart,  lightly  stained  with  hsematoxylin,  and  then 
with  picro-carmine  ;  then  still  further  very  finely  teased 
on  a  slide,  and  mounted  in  glycerin.  The  individual 
fibrillated  fibres  will  now  be  readily  seen,  and  arranged  in 
rows  along  them,  edge  to  edge,  with  more  or  less  irregu- 
lar contours,  the  flattened  tendon  cells  will  be  seen  to 
lie.  Their  nuclei,  stained  violet  by  the  hematoxylin, 
when  seen  from  the  side  appear  oval  or  rectangular,  and 
are  often  arranged  in  couples  along  the  fibres  ;  when 
seen  edgewise  they  look  like  short,  thick  lines.  The 
cell- bodies,  when  seen  sidewise,  look  like  faintly-colored, 
often   indefinitely-outlined,  very    thin    plates,    stretching 


38  NORMAL  HISTOLOGY. 

out  from  the  nuclei,  and  partially  enwrapping  the  tendon- 
fibres.  When  the  cell-body  is  placed  with  its  edge  toward 
the  observer  and  in  close  apposition  with  the  fibre,  it  is 
not  to  be  distinguished,  owing  to  its  extreme  thinness, 
from  the  line  which  marks  the  border  of  the  fibre  ;  if, 
however,  it  has  been  separated,  in  part  or  entirely,  from 
the  fibre  by  the  previous  manipulations,  it  will  appear  as 
a  faint  line,  at  some  part  of  which  is  seen  the  elongated 
nucleus.  Examining  the  specimen  still  further,  one  sees 
in  the  side  view  of  the  cells,  sometimes  along  a  single 
cell,  sometimes  along  a  row  of  them,  one  or  more  fine 
lines  or  striae,  and  on  looking  carefully  at  the  cells  placed 
edgewise,  he  can  convince  himself  that  thin,  narrow 
wings  are  given  off  from  the  cell-body,  and  that  it  is  the 
edges  of  these  wings  which  cause  the  striae  in  the  cells 
seen  on  the  flat.  In  teased  specimens  of  tendon  one  oc- 
casionally finds  irregular-shaped  branching  cells. 

We  have  then  in  tendon  parallel  bundles  of  fibrillated 
fibres,  arranged  along  and  in  the  interstices  between 
which  lie  flattened  cells  whose  thin  bodies  partially  en- 
wrap certain  fibres,  and  send  off  between  other  and 
neighboring  fibres  branches  and  thin  narrow  wings.  The 
interstices  between  the  bundles,  in  which  the  tendon- 
cells  lie,  are  lymph-channels. 

Cells  in  Transverse  Sections  of  Tendon. — A  very  clear 
view  of  the  relations  between  the  cells  and  fibres  of 
tendon,  as  well  as  the  relation  of  the  wings  to  the  cell- 
bodies,  may  be  obtained  by  making  a  transverse  section 
of  a  hardened  and  stained  tendon.  Any  small  tendon 
from  a  recently  killed  animal  will  answer  the  purpose. 
It  should  be  freed  from  surrounding  tissues,  and  imme* 


CONNECTIVE   TISSUE.  39 

diately  immersed  in  a  small  quantity  of  a  half  per  cent, 
solution  of  gold  chloride.  Here  it  remains  for  half  an 
hour,  when  it  is  removed,  washed  with  pure  water,  and 
put  into  a  few  cubic  centimetre  of  the  following  mixture, 
known  as  the  reducing  fluid  : 

Amyl  Alcohol,       .         .  .         .       1 

Formic  Acid,  ...  1 

"Water,  .         .         .         .         .100 

After  remaining  for  twenty-four  hours  in  the  reducing 
fluid,  the  specimen  will  probably  have  assumed  a  rich 
violet  color  ;  if  not,  it  should  be  kept  for  twenty-four 
hours  longer  in  a  fresh  portion  of  the  fluid.  When  the 
requisite  color  has  been  obtained,  the  specimen  is  hard- 
ened for  a  day  in  alcohol,  embedded  in  hardened  liver 
or  wax,  and  thin  cross-sections  made  from  it.  The  sec- 
tions are  now  stained  lightly  with  hematoxylin,  and 
mounted  in  glycerin.  They  should  be  kept  as  much  as 
possible  from  the  light,  which,  after  a  time,  destroys  gold 
preparations.  This  method  of  staining  with  gold  is  known 
as  Pritchard''s  method* 

*  The  older  method  of  gold  staining,  known  as  Conheinis  method, 
differs  from  that  just  given,  in  that,  after  soaking  in  the  gold  solu- 
tion, the  specimen  is  exposed  to  the  light  until  it  has  assumed  the 
violet  color.  It  is  then  mounted  and  preserved  as  before.  This 
method  is  less  certain  in  its  results  than  Pritchard's,  being  less  apt  to 
give  clear  pictures.  In  both  methods  the  staining  action  consists  in 
the  precipitation  of  the  gold  in  metallic  form  in  a  state  of  extreme 
comminution  in  the  tissue.  In  one  case  this  precipitation  is  effected 
by  the  acid  mixture,  in  the  other  by  the  light.  This  deposition  of 
gold  in  the  tissues  occurs,  for  the  most  part,  in  the  protoplasm  of  the 
cells,  bringing  them  into  sharp  contrast  with  surrounding  parts,  and 
fixing  their  form.  There  is  one  source  of  error,  however,  in  the  in- 
terpretation of  gold  specimens,  which  should  never  be  forgotten. 
The  gold  is  apparently  sometimes  deposited,  and  the  characteristic 


40  NORMAL  HISTOLOGY. 

Transverse  Sections  of  the  Cornea. — Cells  seen  from  the 
edge. — V\Te  shall  employ  for  our  study  here  the  cornea  of 
the  frog,  because  of  the  ease  with  which  it  can  be  ob- 
tained, and,  because,  on  account  of  its  thinness,  it  is  well 
suited  to  the  various  manipulations  to  which  we  shall 
subject  it.  A  frog's  eye  should  be  enucleated  directly  after 
death,  and  placed  entire  in  Mtiller's  fluid,  where  it  should 
remain  for  ten  days.  It  "is  then  washed,  and  put,  for  a 
few  hours,  into  dilute  alcohol  (alcohol  one,  water  two), 
then  transferred  to,  and  left,  for  forty-eight  hours,  in 
strong  alcohol.  The  cornea  is  now  excised,  just  within 
the  sclero-corneal  junction,  and,  two  or  three  short 
radial  incisions  having  been  made  at  the  edge,  so  that 
it  will  lie  flat,  it  is  embedded  between  two  bits  of  hard- 
ened liver,  and  thin  transverse  sections  cut  from  it. 
These  are  stained  double,  and  mounted  in  glycerin, 
slightly  tinged  red  with  eosin. 

If  the  sections  are  made  so  as  to  include  the  entire 
thickness  of  the  cornea,  both  the  anterior  and  posterior 
edges  of  the  section  will  be  seen  to  be  covered  with  epi- 
thelial cells.  Between  these  two  layers  of  cells  lies  the 
connective  tissue  substance  of  the  cornea,  which  alone 
concerns  us  here.  This  consists  of  delicate  nbrillated 
fibres,  closely  bound  together  by  a  small  amount  of  ce- 
menting substance,  and  arranged  in  lamellae.  Between 
these  lamellae   are   seen   the   corneal   cells,  which,  in  this 

color  produce  J,  outside  of  the  cctl  protoplasm,  and  especially  in  spaces 
in  the  tissues  which  are  filled  with  albuminoid  materials,  so  that  the 
utmost  caution  is  required  in  determining  whether  a  given  area  of 
gold  deposit  and  staining  really  indicates  the  presence  of  cell  proto- 
plasm. And,  in  fact,  it  is  in  some  cases  impossible,  at  present,  to 
decide  this  question. 


CONNECTIVE   TISSUE.  4 1 

view,  seem  to  have  the  form  of  slender  elongated  spin- 
dles, part  of  them  being  closely  surrounded  by  the  in- 
ter-cellular substance,  part  lying  in  small  elongated  cavi- 
ties. 

Lamina  of  Cornea. — the  Cells  seen  on  the  Flat. — In  order 
to  determine  the  exact  shape  of  the  cells,  which  in  the 
former  preparation  are  seen  only  from  the  edge,  it  is 
necessary  to  look  at  the  cornea  from  the  side.  For  this 
purpose  a  fresh  cornea  should  be  very  carefully  excised, 
all  pulling  or  stretching  of  the  part  being  avoided,  since 
this  distorts  the  cells.  It  is  now  stained  with  gold 
chloride  by  Pritchard's  method,  and  then  laid  on  a  slide 
in  glycerin.  The  epithelium  on  both  the  anterior  and 
posterior  surfaces  is  now  scraped  off,  and  with  a  little 
care  the  part  may  be  divided  into  several  thin  layers  by 
means  of  fine  forceps.  These  layers  are  lightly  stained 
with  hematoxylin  and  mounted  in  glycerin. 

In  a  specimen  thus  prepared,  the  basement  substance 
looks  homogeneous  or  delicately  striated  ;  fine  nerve- 
fibres  are  seen  stretching  across  the  specimen  in  various 
directions.  The  corneal  cells,  which  are  the  most  promi- 
nent objects  in  the  specimen,  are  seen  thickly  scattered 
over  the  field,  stained  of  a  reddish  violet  color.  They 
have  flat,  irregular-shaped  bodies  which  send  off  a  vari- 
able number  of  longer  and  shorter  processes  ;  the  nuclei 
are  large,  ovoidal,  or  irregular-shaped,  and  usually  con- 
tain nucleoli.  Fine  irregular-branching,  almost  linear 
structures  are  seen  in  good  preparations  thickly  scattered 
over  the  specimen,  and  very  frequently  lying  nearly  at 
right  angles  to  one  another.  A  part  at  least  of  these  are 
continuous  with  the  cell-bodies  whose  processes  they  are; 


42  NORMAL  HISTOLOGY. 

whether  or  not  they  are  all  cell  processes  is  not  yet  defi- 
nitely known. 

It  will  be  seen  from  the  above  studies,  that  the  connec- 
tive-tissue cells  of  the  cornea  are  flattened  and  branched 
cells,  and  that  certain  of  them,  at  least,  lie  in  spaces 
between  the  fibres  of  the  intercellular  substance.  We 
have  now  to  study  more  carefully  the  nature  of  these 
spaces  and  their  relation  to  the  cells. 

Cornea  treated  with  Silver. — Relation  of  the  Cells  to 
the  Basement  Substance. — Strong  solutions  of  nitrate  of 
silver  have  the  power  of  staining  the  intercellular  sub- 
stance of  connective  tissue  light  brown,  while  the  cells 
are  left  uncolored.  In  order  to  stain  the  cornea  of  the 
frog  with  silver,  the  following  process  should  be  employed: 
the  spinal  cord  of  the  animal  having  been  broken  up  with 
a  needle,  a  finger  is  introduced  into  the  mouth  so  as  to 
press  the  eyeball  forward  and  bring  the  cornea  into 
prominence  ;  the  membrana  nictitans  is  drawn  away  with 
the  thumb  or  a  pair  of  fine  forceps,  so  as  to  leave  the  an- 
terior surface  of  the  cornea  quite  free.  The  eye  is  now 
held  for  an  instant  over  a  jet  of  steam,  when  the  epithel- 
ium will  become  white  or  milky  in  appearance,  and  can 
be  readily  scraped  off  by  passing  the  blade  of  a  scalpel 
lightly  over  the  surface.  It  is  necessary  to  remove  the 
epithelium  in  order  that  the  silver  solution  may  have 
ready  access  to  the  connective-tissue  substance  of  the 
cornea  ;  and  the  advantage  of  steaming  is,  that  it  can  be 
scraped  off  without  the  use  of  much  force,  which  would 
disturb  the  relations  of  the  parts  beneath.  The  epithe- 
lium being  thus  removed,  a  five-per-cent.  solution  of 
silver  nitrate  is  allowed  to  flow   over  the  cornea  and  re- 


CONNECTIVE   TISSUE.  43 

main  for  two  or  three  minutes  in  contact  with  it.  By 
tin's  treatment  the  whole  cornea  becomes  opaque  and 
stiff.  The  silver  is  now  neutralized  and  its  further  action 
prevented,  by  washing  the  cornea  with  a  one-per-cent. 
salt  solution.  It  is  now  carefully  excised  and  placed  in  a 
dish  containing  a  mixture  of  alcohol  and  water,  one  to 
two,  and  exposed  to  direct  sunlight,  or  bright  daylight, 
for  from  a  few  minutes  to  half  an  hour,  depending  upon 
the  intensity  of  the  light.  When  it  has  become  brown  it 
is  to  be  laid  in  glycerin  and  stripped  into  thin  layers,  as 
directed  above,  for  the  gold  cornea ;  the  layers  are 
mounted  in  glycerin. 

If  the  preparation  is  successful,  clear,  branching,  com- 
municating spaces  are  seen  on  a  yellowish  or  brown 
ground.  These  spaces,  although  larger,  evidently  corre- 
spond in  position  and  in  a  general  way,  in  shape,  to  the 
cells  in  the  cornea,  as  seen  after  treatment  with  chloride 
of  gold  ;  and  if  a  specimen  thus  prepared  be  stained  with 
haematoxylin,  the  corneal  cells  will  be  seen  lying  within 
them. 

It  is  difficult  to  determine  with  certainty  whether  or 
not  the  cells  send  processes  into  all  of  the  channels  which 
radiate  from  the  spaces  in  which  they  lie  ;  it  is  probable 
that  some  of  the  narrow  branching  bodies  or  lines  noticed 
above  in  the  gold  cornea  are  nothing  more  than  these 
branching  and  communicating  spaces  in  which  an  albu- 
minous fluid  accumulates,  which  is  stained  by  the  gold 
very  much  as  cell  protoplasm  itself  is. 

We  thus  see  that  the  cornea  is  permeated  by  numerous 
branching  and  intercommunicating  spaces,  and  that  in 
these  spaces  the  flat,  branching  connective-tissue  cells  lie. 


44  NORMAL  HISTOLOGY. 

These  spaces  are  called  lymph-spaces,  and  what  we  have 
been  able  to  demonstrate  in  regard  to  the  relation  of  cells 
to  the  lymph-spaces  in  the  cornea,  by  these  various  modes 
of  preparation,  seems  to  be  true,  with  certain  modifica- 
tions, of  the  cells  in  most  of  the  varieties  of  connective 
tissue.  These  cells  lie  in  spaces,  sometimes  completely, 
sometimes  only  partially  filling  them.  These  spaces  com- 
municate with  one  another,  and  communicate  also,  on  the 
one  hand,  with  the  blood-vessels,  and,  on  the  other,  with 
the  lymphatics.  Through  them  lymph-currents  pass, 
bathing  the  cells  and  supplying  them  with  nutritive  ma- 
terial. 

It  is  extremely  probable  that  it  is  through  these  lymph 
spaces  exclusively  that  the  white  blood-cells  travel,  in 
their  perigrinations  through  the  tissues.  We  study  them 
in  the  cornea  alone,  because  the  scope  of  this  manual  is 
too  limited  to  admit  of  such  a  detailed  study  of  all  vari- 
eties of  connective  tissue,  and  because  here  the  relation 
of  the  cells  to  the  spaces  is  clearly  defined. 

Endothelium  of  the  Serous  Membranes. — The  serous 
membranes  are  formed  by  layers  of  fibrillar  connective 
tissue  fibres  mingled  with  a  varying  number  of  elastic 
fibres,  and  containing  ordinary  flattened  connective-tissue 
cells.  They  are  more  or  less  abundantly  furnished  with 
blood  and  lymphatic  vessels.  On  the  free  surface  of 
these  membranes  rests  a  continuous  layer  of  flattened 
cells,  differing  in  many  respects  from  the  ordinary  con- 
nective-tissue cells,  and  called  endothelial  cells  or  endothel- 
ium. These  cells  are  usually  transparent,  irregularly 
polygonal  in  form,  and  frequently  much  elongated  ;  they 
possess  one  or  more  ovoidal  nuclei,  which  often  project 


CONNECTIVE   TISSUE.  45 

above  the  general  level  of  the  free  surface  of  the  cell- 
body  ;  they  arc  placed  edge  to  edge,  like  the  stones  in  a 
mosaic,  and  seem  to  be  joined  together  by  a  minimal 
amount  of  an  albuminoid  cement  substance." 

Endothelium  covering  the  Mesentery. — In  order  to  bring 
the  outlines  of  the  cells  clearly  into  view,  the  membrane 
should  be  first  treated  with  a  solution  of  nitrate  of  silver. 
This  substance  in  dilute  solutions  forms  with  the  cement 
substance  between  the  endothelium  an  albuminate  of  sil- 
ver, which,  on  exposure  to  light,  becomes  brown  or  black, 
thus  clearly  denning  the  outline  of  the  cells. 

The  mode  of  proceeding  is  as  follows  :  A  portion  of 
the  mesentery  of  the  recently  killed  dog  or  rabbit  should 
be  carefully  removed  and  laid  over  the  rim  of  a  shallow 
dish  so  that  it  rests  loosely  over  the  opening.  All  stretch- 
ing and  pulling  of  the  membrane  should  be  avoided,  be- 
cause this  would  destroy  the  natural  relations  of  the  ceils 
to  one  another.  The  membrane  must  be  allowed  to  sag 
a  little  into  the  dish,  so  that  it  may  be  bathed  on  both 
sides  by  the  silver  solution.  It  is  now  to  be  carefully 
washed  with  water  to  remove  any  albuminous  substance 
or  blood — which  would  cause  a  granular  precipitate  of 
silver  albuminate  on  the  surface — and  the  dish  then  filled 
with  an  aqueous  solution  of  nitrate  of  silver,  i  to  500. 
The  dish  should  be  gently  shaken  at  frequent  intervals, 
so  as  to  bring  fresh  portions  of  the  soLution  into  con- 
tact with  the  membrane,  and  after  from   twenty  minutes 

*  There  is  reason  for  believing  that  the  cement  substance  between 
the  endothelium,  as  well  as  between  many  kinds  of  epithelial  ceils  15 
permeable  to  fluids,  and,  under  certain  conditions,  to  solid  pr.rticles 
also,  and  thus  forms  an  avenue  of  communication  between  adjacent 
but  separaied  cavitie-  or  lymph  -paces. 


46  ORMAL  HISTOLOGY. 

to  half  an  hour  the  tissue  will  be  seen  to  have  become 
cloudy  or  milky. 

The  silver  is  now  poured  off,  and  the  membrane  care- 
fully washed  with  water.  The  cells  will  have  been  fixed 
by  the  silver,  so  that  the  membrane  may  be  removed 
without  further  danger  of  disturbing  the  relation  of  the 
cells,  and  laid  in  a  dish  containing  water  to  which  one- 
third  its  bulk  of  alcohol  has  been  added.  It  is  now  ex- 
posed to  the  sunlight,  and  after  from  a  few  minutes  to 
half  an  hour — sometimes  longer,  depending  upon  the  in- 
tensity of  the  light — the  tissue  will  be  seen  to  have  as- 
sumed a  brown  color. 

A  small  piece  from  the  thinner  portion  should  now  be 
lightly  stained  with  hsematoxylin  and  mounted  in  glyc- 
erin. 

These  portions  of  the  mesentery  consist  of  a  thin  mem- 
brane of  fibrillar  connective  tissue  containing  a  delicate 
network  of  fibres  and  a  few  small  blood-vessels  ; 

the  whole  being  covered  on  both  sides  by  the  delicate 
mosaic  of  endothelial  ceils.  The  outlines  of  these  cells 
on  both  sides,  may  be  brought  successively  into  view  by 
careful  focussing. 

Endothelium  of  the  Omentum. — A  portion  of  the  omen- 
tum of  the  dog  should  be  treated  with  silver  in  the  way 
directed  for  the  mesentery,  and  also  stained  with  he- 
matoxylin and  mounted  in  glycerin.  The  omentum  of 
the  dog,  as  of  man,  consists  of  an  irregular  meshed  net, 
whose  trabecular  are  made  up  of  fascicles  of  fibrillar 
connective  tissue  of  varying  thickness,  the  broader  con- 
taining blood-  and  lymph-vessels  and  fat-cells,  the  nar- 
:ing   of  single  bundles  of  fibrillar — all  being 


CONNECTIVE   TISSUE.  47 

alike  covered  with  a  single  layer  of  endothelium.  In 
many  parts  the  endothelial  cells  are  seen  in  profile,  when 
their  nuclei  appear  lenticular  in  form,  usually  projecting 
above  the  general  level  of  the  cell-body,  which  is  itself 
in  most  cases  thicker  in  the  vicinity  of  the  nucleus  than 
at  the  points  of  junction  with  adjoining  cells.  The  speci- 
mens of  both  omentum  and  mesentery  may  be  preserved 
in  the  usual  Avay  ;  but  silver  preparations,  like  the  gold, 
do  not  preserve  their  first  clearness  very  long,  unless  care- 
fully kept  from  the  light. 


CHAPTER  III. 

EMBRYONAL    AND    MUCOUS    TISSUE.— FAT    TISSUE.— 
RETICULAR  CONNECTIVE  TISSUE. 

EMBRYONAL    AND    MUCOUS    TISSUE. 

At  a  certain  period  of  embryonic  life,  those  parts 
of  the  body  which  are  destined  finally  to  become 
fibrillar  connective  tissue,  consist  almost  entirely  of 
small  spheroidal  cells — the  intercellular  substance 
being  absent,  or  consisting  only  of  a  very  small 
quantity  of  fluid  lying  between  and  bathing  the 
cells.  As  the  process  of  development  goes  on,  some 
of  the  cells  retain  their  original  spheroidal  form, 
while  others  change  their  character,  becoming  elon- 
gated and  fusiform,  often  terminating  at  their  ex- 
tremities in  delicate  single  or  branching  processes ; 
others  become  flattened  and  assume  irregular 
shapes,  sending  off  branching  processes  by  which 
they  are  joined  to  one  another.  Hand  in  hand  with 
this  change  in  the  cells  there  occurs  an  accumula- 
tion of  intercellular  material,  which  is  at  first  fluid, 
and  later  presents  the  appearance  of  a  homogene- 
ous gelatinoid  substance.  Then  within  the  gelati- 
noid  intercellular  substance  appear  fine  fibrillar, 
which  become  more  and  more  abundant,  arranging 
themselves  now  in  bundles,  and  again  to  form  irregu- 


EMBRYONAL  AND  MUCOUS  TISSUE.  49 

lar  networks.  The  cells  approach  more  and  more 
closely  to  the  type  of  the  adult  connective-tissue 
cells,  as  development  goes  on  ;  the  intercellular  sub- 
stance loses  its  soft  gelatinoid  character,  and  is 
finally  replaced  by  the  fibrillated  and  elastic  fibres 
with  which  we  are  already  familiar. 

The  process  of  development  is  a  very  gradual 
one,  and  although  the  younger  forms  of  embryonal 
connective  tissue  and  the  adult  connective  tissue 
are  distinct  enough,  we  are  yet  unable  to  separate 
them  sharply,  since  they  merge  so  gradually  into 
one  another.  In  general,  however,  simply  as  a  mat- 
ter of  convenience,  we  call  connective  tissue  which 
is  almost  entirely  made  up  of  spheroidal,  spindle- 
shaped,  or  flattened  cells,  in  which  little  accumula- 
tion and  little  differentiation  of  the  intercellular 
substance  has  occurred,  embryonal  tissue ;  while  to 
that  older  form,  which  consists  of  variously  shaped, 
spheroidal,  flat,  branching  and  anastomosing  cells, 
with  a  gelatinoid,  homogeneous,  or  partially  fibri- 
lated  intercellular  substance,  the  term  mucous  tissue 
is  usually  applied.  The  name  mucous  tissue  was 
given  to  this  form  of  young  tissue,  because  the  soft 
gelatinoid  intercellular  substance-was  found  to  con- 
tain a  certain  amount  of  mucin,  which  may  be 
thrown  down  in  the  form  of  a  whitish,  often  stringy 
precipitate,  by  the  addition  of  acetic  acid.  At 
present,  however,  tissues  presenting  the  above 
mentioned    morphological    characters    are    usually 


$0  NORMAL  HISTOLOGY. 

called  mucous  tissues,  whether  the  intercellular  sub- 
stance contains  mucin  or  not.  Mucous  tissue  is  not 
found  in  the  health}-  human  adult,  but  frequently 
occurs  under  pathological  conditions. 

PRACTICAL   STUDY. 

Subcutaneous  Tissue  of  Embryo. — Embryonal  connec- 
tive tissue  may  be  studied  in  any  young  mammalian  em- 
bryo, preserved  in  Miiller's  fluid  and  alcohol.  Bits  of  the 
subcutaneous  tissue  are  torn  off  from  the  abdominal  wall, 
stained  double,  finely  teased,  and  mounted  in  glycerin. 

Mucous  Tissue  from  the  Umbilical  Cord. — In  the  um- 
bilical cord  of  a  nearly  mature  foetus  we  find  typical  mu- 
cous tissue.  A  bit  of  the  cord  of  any  mammalian  foetus, 
as  the  pig,  is  hardened  in  Midler's  fluid  and  alcohol,  then 
embedded  in  liver  and  transverse  sections  made.  These 
are  stained  double  and  mounted  in  clvcerin  or  balsam. 
The  surface  of  the  cord  is  seen  to  be  covered  with  lami- 
nated epithelium,  and  the  three  large  blood-vessels  are 
seen  to  be  cut  across.  The  amount  of  fibrillation  present  in 
the  intercellular  substance  depends  upon  the  age  of  the 
foetus. 

FAT    TISSUE. 

Fat-tissue  is  a  modified  form  of  connective  tissue, 
in  which  the  intercellular  substance  is  present  in 
proportionally  small  amount,  and  a  large  part  of  the 
protoplasm  of  the  cells  is  replaced  by  fat,  which 
crowds  the  remaining  part,  together  with  the  nu- 
cleus, to  the  side  of  the  cell,  nearly  or  entirely  con- 


FAT  TISSUE.  51 

cealingboth.  The  fat-cells  thus  formed  are  arranged 
in  clusters  or  lobules,  enclosed  by  fibrillar  connective 
tissue,  which  sends  into  the  lobules  and  between 
the  cells  broader  and  narrower  bundles,  which  serve 
to  support  the  cells  and  carry  the  blood  and  lym- 
phatic vessels,  etc.  Owing  to  the  pressure  to  which 
the  fat-cells  are  subjected,  they  usually  assume,  in 
the  adult  animal,  a  polyhedral  form. 

Sometimes  the  fat  appears  within  the  cell  in  the 
form  of  clusters  of  radiating  needle-like  crystals. 

In  order  to  understand  clearly  the  nature  of  adult 
fat-tissue,  it  is  necessary  to  study  it  during  the  pro- 
cess of  development.  At  an  early  period  of  life, 
those  parts  of  the  body  which  are  finally  to  become 
fat-tissue,  possess  the  character  of  mucous  tissue 
with  a  more  or  less  fibrillated  intercellular  substance. 
The  first  change  which  we  notice  in  the  cells  as  the 
transformation  into  fat-tissue  commences,  is  the  ap- 
pearance in  the  cell-body  of  small  shining  particles. 
These  particles,  of  which  there  may  be  many,  be- 
come gradually  larger,  until  they  present  the  form 
and  character  of  distinct  droplets  of  fat.  As  these 
droplets  increase  in  size,  they  coalesce,  forming  one 
or  more  drops,  which  presently  become  large  enough 
to  crowd  the  nucleus  to  one  side.  The  growing 
drops  finally  unite  into  one  large  drop,  which  at 
length  becomes  so  large  as  to  occupy  nearly  the 
whole  of  the  cell,  leaving  only  a  thin  crescent  of  pro- 
toplasm   and    a   squeezed    and    distorted    nucleus 


52  NORMAL  HISTOLOGY. 

crowded  up  against  the  cell-membrane.  At  last  we 
can  no  longer  see,  without  special  modes  of  prepara- 
tion, any  trace  of  cell  protoplasm — although  a  small 
amount  of  this  really  persists  as  a  thin  shell  within 
the  membrane — and  only  the  deformed  remnant  of 
the  nucleus.  This  process  is  called  fatty  infiltra- 
tion. 

In  those  parts  of  the  body  where  the  fat  is  invari- 
ably found,  this  change  in  the  cells  occurs  in  the 
vicinity  of  little  tufts  of  capillary  blood-vessels,  so 
that  at  one  period,  the  forming  fat  is  seen  lying  in 
scattered  clusters  in  the  meshes  of  distinct  groups 
of  blood-capillaries.  It  is  these  clusters  of  fat-cells, 
with  their  accompanying  blood-vessels,  which  deter- 
mine, when  the  fat  is  fully  formed,  the  lobular  char- 
acter of  this  tissue. 

In  many  parts  of  the  body  and  under  varying 
conditions — sometimes  physiological,  sometimes 
pathological — there  is  an  accumulation  of  fat  in  the 
protoplasm  of  cells;  but  it  is,  under  normal  condi- 
tions for  the  most  part,  temporary,  and  the  fat- 
cells  have  no  definite  grouping  in  lobules  and  about 
the  blood-vessels  in  the  way  above  described  for  the 
permanent  fat. 

PRACTICAL    STUDY. 

Fresh  Fat,  teased. — A  bit  of  fresh  subcutaneous  fat 
should  be  teased  and  covered  in  salt  solution,  without 
staining.     In   such  a  specimen  little  else  is  seen  than  a 


FAT  TISSUE.  53 

congeries  of  more  or  less  globular  or  polyhedral  masses 
of  fat-cells,  with  larger  and  smaller  free  fat  droplets.  The 
fat  is  characterized  by  its  great  refractive  power  ,  by  the 
dark  contour  of  its  globules  by  transmitted,  and  their  yel- 
lowish silvery  lustre  by  reflected,  light  ;  by  its  solubility  in 
alcohol  and  ether,  and  by  the  deep  black  color  which  it 
acquires  on  treatment  with  osmic  acid.  The  connective 
tissue  which  encloses  the  lobules  is  seen  in  broken  masses 
and  shreds,  scattered  through  the  preparation. 

Section  of  Adult  Fat  2Yssue. — A  small  piece  of  subcuta- 
neous fat  from  man  should  be  hardened  in  alcohol,  by 
which  the  fat  will  be  for  the  most  part  dissolved  out  of 
the  cells.  Thin  sections  are  made,  stained  double,  and 
mounted  in  balsam.  This  preparation  shows  the  relation 
of  the  cells  to  one  another,  and  the  lobular  structure  of 
the  tissue.  If  the  blood-vessels  of  the  part  from  which 
the  tissue  is  taken  have  been  previously  injected  with  the 
blue  gelatin  mixture,  the  relations  of  the  vessels  to  the 
lobules  and  cells  will  be  well  shown. 

Developing  Fat  from  Young  Animals. — To  study  this  in 
its  early  stages,  some  of  the  fresh  mucous  tissue  from  the 
axilla  or  groin  of  a  fcetal  animal,  such  as  a  pig,  5  inches 
long,  should  be  immersed  for  24  hours  in  i-per-cent.  osmic 
acid  ;  then  washed  and  teased,  and  mounted  in  glycerin 
slightly  tinged  with  eosin.  The  larger  and  smaller  fat 
droplets  within  the  cells  will  be  black,  and  numerous  cells 
will  be  seen  in  which  the  fatty  infiltration  has  not  com- 
menced. 

To  study  the  fat  cells  at  a  later  period  of  development, 
thin  sections  may  be  made  from  the  subcutaneous  fat  of 
the  human   foetus,  at  from   6  to  3   months,  or  from  any 


54  NORMAL  HISTOLOGY. 

mammal  of  corresponding  age.  These  are  stained  double 
and  mounted  in  balsam.  At  this  period,  while  the  fat 
droplet  in  many  parts  occupies  the  greater  part  of  the 
cell,  a  distinct  crescentic  mass  of  protoolasm  is  still  seen 
at  one  side,  enclosing  the  nucleus. 

RETICULAR    CONNECTIVE    TISSUE. 

This  tissue  forms  a  large  part  of  the  supporting 
framework  of  the  lymphatic  glands,  and  is  found,  in 
somewhat  modified  form,  in  other  parts  of  the  body. 
It  consists  of  delicate  fibres,  of  varying  diameter, 
which  cross  and  join  one  another  at  frequent  inter- 
vals, forming  a  fine  meshed  network.  This  net- 
work of  fibres  is  not  flattened  to  form  a  membrane, 
but  extends  in  all  directions,  like  the  trabecular  of  a 
sponge.  Irregularly  scattered  over  the  fibres  are 
flattened  nucleated  cells,  having  the  character  of 
endothelium,  which  sometimes  lie  at  the  points  of 
intersection  of  the  fibres,  sometimes  along  their 
sides,  enwrapping  them  with  their  transparent 
bodies.  When  these  cells  are  in  situ  upon  the  fibres, 
the  whole  presents  the  appearance  of  a  mass  of  an- 
astomosing, branched,  or  spindle-shaped  cells  ;  and, 
as  such,  the  reticular  connective  tissue  has  until  re- 
cently been  regarded, — erroneously,  however,  as  is 
shown  by  the  fact  that  by  appropriate  manipulation 
the  flat  cells  can  be  entirely  freed  from  the  underly- 
ing fibre-net,  leaving  the  latter  intact.  The  meshes 
of  the  reticular  tissue  are  loosely  filled,  in  the  lym- 


RETICULAR   CONNECTIVE   TISSUE.  55 

phatic  glands,  with  small  spheroidal  cells — lymph 
cells — which,  however,  seem  to  have  \\o  direct  con- 
nection with  the  tissue  we  are  studying,  and  may  be 
easily  removed. 

PRACTICAL    STUDY. 

Section  of  the  Lymphatic  Gland  of  Dog,  Treated  with  Osmic 
Acid. — One  of  the  mesenteric  or  cervical  glands  is  re- 
moved from  a  recently-killed  dog,  and  a  hypodermic 
syringe  being  partially  filled  with  i-per-cent.  solution  of 
osmic  acid,  the  ca.nula  is  thrust  well  into  the  gland,  and 
the  acid  slowly  injected  until  the  organ  becomes  quite 
tense  ;  the  canula  is  then  withdrawn,  and  the  gland  placed 
in  strong  alcohol.  After  a  few  days  it  will  be  hard 
enough  to  make  sections  from.  The  sections,  which  must 
be  very  thin,  should  be  carefully  shaken  in  a  test-tube, 
one- third  filled  with  water,  to  remove  the  lymph  cells 
which  lie  in  the  meshes  of  the  reticular  tissue  and  conceal 
it.  As  these  cells  are  shaken  out,  the  sections  look  thin- 
ner, and  when  the  operation  is  completed  they  are  stained 
with  hematoxylin  and  mounted  in  glycerin. 


CHAPTER  IV. 

CARTILAGE— BONE— TEETH. 

CARTILAGE. 

Cartilage  consists,  like  other  members  of  the 
lective-tissue  group,  of  cells  and  intercellular 
substance.  There  is  nothing  characteristic,  how- 
.  in  the  form  of  the  cartilage-cells.  It  is  in  the 
peculiar  nature  of  the  intercellular  substance  and 
the  relations  which  the  cells  bear  to  it,  that  we  find 
the  distinctive  features  of  this  form  of  connective 
tissue.  The  cartilage-cells  are  spheroidal,  flattened, 
or  angular  in  form  ;  the  cell-body  is  finely  granular 
and  often  contains  tiny  droplets  of  fat,  and  some- 
times pigment-granules.  The  cells  have  one,  or 
sometimes  two,  sharply  defined  nuclei,  which  are 
coarsely  granular  and  often  contain  an  irregular 
network  of  a  more  strongly  refractive  substance. 
Around  each  cartilage-cell,  in  the  adult  animal,  and 
closely  enclosing  it,  is  a  homogeneous  envelope 
called  the  capsule.  The  substance  forming  this  cap- 
sule is  identical  with  the  intercellular  substance  of 
hyaline  cartilage,  presently  to  be  described. 

The  cartilage-cells  very  readily  lose  their  normal 
form  and  relation  to  the  capsule  by  the   application 

56 


CARTILAGE.  5/ 

of  a  great  variety  of  substances,  and  even  by  slight 
exposure  to  the  air.  The  most  common  change 
which  is  noticed  in  them  is  a  rapid  shrinkage,  such 
as  occurs  when  cartilage  is  exposed  to  the  air  or 
treated  with  strong  salt  solutions,  or  any  substance 
which  extracts  water  from  the  •  tissues.  Under 
these  circumstances  the  cell  becomes  more  coarsely 
granular,  it  shrinks  away  from  the  capsule  at  certain 
points,  giving  the  edge  of  the  cell  a  festooned  ap- 
pearance;  sometimes  large,  clear  spheroidal  spaces, 
called  vacuoles,  appear  in  the  cell-body,  and  finally 
the  cell  shrinks  to  a  shapeless  mass  in  the  centre  or 
at  one  side  of  the  cavity,  or  retains  its  connection 
with  the  capsule  by  one  or  more  narrow  irregular 
projections  from  the  shrunken  mass. 

The  basement  or  intercellular  substance  is  not 
the  same  in  all  cartilages,  and,*  according  to  the  dif- 
ferences in  its  nature,  cartilage  is  divided  into  hya- 
aline  cartilage,  fibro-cartilage,  fibro-elastic  cartilage. 

In  hyaline  cartilage  the  intercellular  substance  is 
homogeneous  and  transparent  in  thin  layers,  some- 
what opalescent  in  thicker  masses ;  it  is  of  firm 
consistence,  and  contains  at  tolerably  regular  inter- 
vals variously  shaped  cavities  in  which  the  cells  lie, 
exactly  filling  them.  The  layer  of  basement  sub- 
stance which  immediately  surrounds  the  cells  pos- 
sesses slightly  different  refractive  power,  and  it  is 
this  layer  which  constitutes  the  capsule  above  men- 
mentioned. 


5S  NORMAL  HISTOLOGY. 

The  cell-spaces  or  cavities  in  hyaline  cartilage  do 
not  by  the  ordinary  modes  of  preparation  appear 
to  be  connected  by  lymph-channels,  as  are  the 
cell-spaces  in  other  varieties  of  connective  tissue, 
yet  the  rapid  passage  of  fluids  through  the  tissue 
under  certain  circumstances,  together  with  some 
recent  microscopical  observations,  renders  it  ex- 
tremely probable  that  such  communications  do 
exist,  though  we  are  not  at  present  able  to  demon- 
strate them  with  certainty.  Although  by  the  ordi- 
nary modes  of  preparation  the  basement  substance 
of  hyaline  cartilage  appears  quite  homogeneous, 
certain  changes  which  it  undergoes  under  patholog- 
ical conditions,  or  by  the  use  of  certain  macerating 
or  digesting  fluids,  lead  us  to  believe  that  it  really 
contains  a  groundwork  of  delicate  fibrillae. 

The  basement  substance  of  hyaline  cartilage  dif- 
fers chemically  from  the  basement  substance  of 
other  members  of  the  connective-tissue  group,  yet 
recent  researches  have  thrown  serious  doubt  upon 
the  view  formerly  held,  i.  e.t  that  cartilage  gave,  on 
boiling,  a  peculiar  and  characteristic  substance 
called  chondrin,  it  having  been  shown  that  the  so- 
called  chondrin  is  not  a  pure  chemical  substance, 
but  a  mixture  of  gelatin,  mucin,  and  certain  salts. 

Hyaline  cartilage  is  found,  in  the  adult,  covering 
the  ends  of  the  bones  in  the  joints ;  and  most  of  the 
laryngeal  and  the  tracheal,  bronchial,  costal  and  nasal 
cartilages  are  of  this  variety. 


CARTILAGE.  59 

Fibro-cartilage  differs  from  hyaline  cartilage  in 
having  a  distinctly  fibrillated  intercellular  substance. 
This  form  of  cartilage  is  found  in  the  intervertebral 
cartilages,  in  the  meniscuses  of  certain  joints,  and 
at  certain  points  where  the  ligaments  are  inserted 
into  the  cartilaginous  extremities  of  bones. 

In  certain  other  cartilages — such  as  the  epiglottis, 
some  of  the  small  cartilages  of  the  larynx,  and  the 
cartilage  of  the  pinna  of  the  ear — the  intercellular 
substance  contains,  in  addition  to  a  few  fibrillated 
fibres,  elastic  tissue,  either  in  the  form  of  fibres  or 
in  fine  granules.  Such  cartilage  is  called  fibro-elastic 
cartilage.  In  both  fibro-  and  fibro-elastic  cartilage 
the  cells  are  identical  in  character  with  those  of 
hyaline  cartilage. 

Except  at  the  free  surfaces,  which  it  presents  in 
the  joints,  cartilage  is  surrounded  by  a  layer  of 
fibrillated  connective  tissue  of  varying  'thickness, 
called  the  perichondrium ',  in  which  are  found  the 
blood-vessels  which  supply  nutritive  material  to  the 
non-vascular  cartilage  within.  At  the  surface  of 
many  cartilaginous  masses  the  ceils  are  very  much 
crowded  together,  and  flattened  in  a  plane  parallel 
to  the  surface. 

PRACTICAL    STUDY. 

Hyaline  Cartilage  from  Femur  of  Frog. — The  head  of 
a  femur  of  a  recently  killed  animal  being  exposed,  a  thin 
slice  of  cartilage  is   shaved  off  with  a  razor  so  as  to  leave 


6o  NORMAL  HISTOLOGY. 

a  flat  surface  from  which  a  thin  section  should  be  cut, 
and  immersed  in  a  drop  of  saturated  solution  of  picric 
acid  on  a  slide,  and  covered  and  surrounded  at  once  by 
a  rim  of  asphalt  varnish,  before  the  acid  solution  com- 
mences to  evaporate. 

In  such  thin  sections,  cavities  are  seen  here  and  there 
from  which  the  cells  have  fallen  out  ;  these  may  be  filled 
w.th  the  preservative  fluid,  or  with  bubbles  of  air.  Picric 
acid  is  one  of  the  best  agents  for  preserving  the  normal 
characters  of  cartilage  cells,  but  even  in  this  they  shrink 
somewhat,  and  after  a  time  become  coarsely  granular. 

Shrinkage  of  Cartilage  Cells  by  Strong  Salt  Solution. — 
Since  in  most  of  the  specimens  containing  cartilage, 
which  we  shall  study  later,  the  cells  will  have  been  con- 
siderably shrunken  by  the  preservative  fluids,  it  is  desira- 
ble for  us  to  acquaint  ourselves  here  with  the  process  of 
shrinkage,  so  that  we  may  understand  the  various  dis- 
torted forms  which  such  cells  present.  To  do  this,  we 
have  simply  to  prepare  a  thin  section  of  hyaline  cartilage, 
as  above,  and  mount  it  at  once  in  half-per-cent.  salt  solu- 
tion. When  a  good  field  for  observation  has  been  se- 
lected, a  drop  of  ten-per-cent.  salt  solution  (alcohol  will 
do  as  well)  is  run  in  at  one  edge  of  the  cover  glass,  the 
agent  being  drawn  through  by  filter  paper  at  the  other. 
In  this  way  the  process  of  shrinkage  and  formation  of 
vacuoles  may  be  readily  observed. 

Fibro-cariilage  may  be  studied  in  thin  sections  from 
the  intervertebral  cartilages  of  man  or  any  of  the  domes- 
tic animals  ;  or  from  the  head  of  the  femur,  through  the 
insertion  of  the  ligamentum  teres,  parallel  with  the  course 
of  its  fibres.     The  tissues  should  be  laid  for  a  few  days  in 


BOXE.  6 1 

alcohol  before  cutting.  The  sections  are  stained  with 
'-carmine,  and  mounted  in  glycerin. 
Fibro-elastic  Cartilage, — This  is  best  studied  in  the 
epiglottis  of  man  or  the  lower  animals,  which  has  been 
preserved  in  alcohol.  Thin  sections  are  stained  with 
picro-carmine,  which  colors  the  cells  red,  and  the  elastic 
granules  and  fibres  yellow.  They  are  mounted  and  pre- 
served in  glycerin.  The  cells  in  both  fibro-  and  fibro- 
elastic  cartilage  are,  by  the  above  modes  of  preparation, 
more  or  less  shrunken  and  deformed. 

BONE. 

In  studying  bone  we  have  to  consider:  I,  the 
hard  substance,  or  bone-tissue  proper ;  2,  the  connec- 
tive tissue  envelope  which  surrounds  the  bone — the 
periosteum ;  and,  3,  the  marrow  contained  in  the 
central  cavities  or  spaces  within. 

1.  The  most  striking  feature  of  bone-tissue  proper 
is  its  firmness  and  hardness,  which  is  due  to  the 
presence  of  certain  inorganic  salts  of  lime.  Various 
acids  dissolve  these  lime  salts,  and  when  they  are 
removed  a  substance  is  left  behind  which  retains, 
for  the  most  part,  both  in  general  form  and  minute 
structure,  all  the  essential  features  of  the  original 
bone.  There  is  a  basement  substance,  presenting 
many  of  the  optical  characters  of  hyaline  cartilage ; 
and  lying  in  tiny,  branching  spaces,  in  the  basement 
substance,  are  flat,  nucleated  cells.  The  lime  salts 
are  deposited  in  the  intercellular  substance  in  such 


62  NORMAL  HISTOLOGY. 

an  extremely  minute  state  of  division  as  to  be  in- 
visible, even  with  high  powers  of  the  microscope. 
The  elongated  and  flattened  cell-spaces  of  bone  are 
frequently  called  lacuna,  and  the  numerous  fine, 
branching,  intercommunicating  channels  which  pass 
off  from  them  in  all  directions,  and  open  into  the 
marrow  cavities,  or  into  the  passages  for  the  blood- 
vessels, are  called  canaliculi. 

The  bone-cells,  or  bone-curpuscies,  as  they  were  for- 
merly called,  are,  in  adults,  thin,  flat  cells,  with  sphe- 
roidal or  oval  projecting  nuclei.  It  was  formerly  be- 
lieved that  they  sent  fine  branching  processes  off  into 
the  ramifications  of  the  canaliculi.  Recent  investiga- 
tions, however,  have  thrown  great  doubt  upon  the 
existence  of  at  least  such  numerous  processes  as 
were  formerly  believed  to  exist — it  having  been 
shown  that  some,  at  least,  if  not  all,  of  the  sup- 
posed processes  are  really  portions  of  the  intercel- 
lular substance  lining  the  lacunae  and  canaliculi. 
In  young  bone  the  cells  are  not  flat,  but  spherical 
or  ovoidal. 

We  distinguish  two  kinds  of  bone-tissue,  spongy 
and  compact. 

In  spongy  bone-tissue,  which  is  found  in  abundance 
in  the  epiphyses  of  the  long  bones,  the  hard  sub- 
stance, or  bone  proper,  is  arranged  in  the  form  of 
thin  plates,  which  are  grouped  together  so  as  to 
enclose  tiny,  irregular-shaped  cavities,  filled  with 
marrow-tissue.       In    these    thin    plates    of    spongy 


BONE.  63 

bone  the  cells  lie  irregularly  scattered  through  the 
intercellular  substance,  which  is  homogeneous. 

In  compact  bone,  such  as  is  found  in  the  diaphyses 
of  the  long  bones,  the  intercellular  substance  is  ar- 
ranged in  layers,  or  lamellae,  in  and  between  which 
lie  the  cells.  The  lamellar  arrangement  is  best  seen 
in  transverse  sections  from  the  diaphyses  of  the  long 
bones.  If  we  look  at  a  thin  cross-section  of  such  a 
bone  with  a  low  magnifying  power,  we  notice  nu- 
merous round  or  ovoid,  or  irregular-shaped  open- 
ings, of  varying  size,  and  around  these  are  grouped 
several  thin  concentric  layers  of  basement  substance, 
in  and  between  which  lie  the  cells,  flattened  in  the 
plane  of  the  lamellae. 

These  sets  of  concentric  lamellae  are  called  special 
or  Haversian  systems  of  lamcllce.  Sometimes  these 
Haversian  systems  of  lamellae  lie  closely  crowded 
together,  and  again  they  lie  at  varying  distances 
from  one  another.  In  the  latter  case  the  interven- 
ing space  is  filled  up  by  other  and  irregular  sets  of 
lamellae  which  do  not  correspond  with  the  Haversian 
lamellae,  but  pass  off  obliquely  in  various  directions. 
These  are  called,  from  their  position  relative  to  the 
Haversian  system,  intermediate  lamcllce.  Finally,  at 
the  surface  of  the  bone  beneath  the  periosteum,  and 
sometimes  at  the  inner  surface  adjacent  to  the  me- 
dullary cavity,  are  seen  a  series  of  lamellae  which  lie 
parallel  to  the  surfaces  of  the  bone,  and  are  called 
general  or  circumferential  lamcllce.     If  we  make  a 


64  NORMAL  HISTOLOGY. 

longitudinal  section  of  a  long  bone,  we  find  that  it 
is  traversed  by  a  number  of  more  or  less  longi- 
tudinally arranged,  branching  and  communicating 
canals,  of  varying  size,  in  which  lie  the  blood-  and 
lymph-vessels.  It  is  around  these  canals,  called 
Haversian  canals,  that  the  Haversian  lamellae  are 
grouped,  and  the  variously  shaped  openings  which 
are  seen  in  the  transverse  sections,  are  transverse 
sections  of  these  vascular  or  Haversian  canals. 
Within  the  Haversian  canals,  when  they  are  not  en- 
tirely filled  with  the  blood-vessels,  we  find  the  latter 
enclosed  in  a  tissue  identical  with  that  filling  the 
medullary  cavity,  and  presently  to  be  described  as 
marrow.  In  the  flat  and  irregular-shaped  bones 
essentially  the  same  structural  features  are  present, 
but  the  lamellar  arrangement  is  much  less  regular. 

Although,  for  the  most  part,  the  intercellular  sub- 
stance of  bone  is,  by  the  ordinary  modes  of  prepa- 
ration, apparently  quite  destitute  of  structure  be- 
yond that  indicated  by  its  lamellation,  we  yet  find 
in  certain  portions  a  well-defined  system  of  fibres. 
If,  in  a  decalcified  bone,  some  of  the  external  la- 
mellae are  torn  off,  numerous  fine,  fibrillated,  spicula- 
like  projections  are  seen  hanging  on  to  the  inner 
surface  of  the  separated  fragments.  These  are  the 
so-called  Sliarpey  s  fibres,  which,  passing  inward  from 
the  periosteum,  pierce  the  bone  either  obliquely  or 
at  right  angles.  As  we  shall  see  when  studying  the 
development  of  bone,  these  Sharpey's  fibres  are  the 


BONE.  65 

remains  of  fibrillated  connective-tissue  bundles, 
which  originally  occupied  the  situation  now  filled 
by  bone.  Recent  investigations,  moreover,  have 
led  to  the  belief  that  in  bone,  as  in  hyaline  cartilage, 
the  basement  substance  is  everywhere  delicately 
fibrillated,  but  we  have  not  space  in  this  manual  to 
consider  the  methods  by  which  this  may  be  demon- 
strated. 

2.  Periosteum. — The  periosteum  consists  chiefly 
of  fibrillated  connective  tissue,  with  a  few  elastic 
fibres,  and  we  recognize  in  it  two  layers  :  an  outer 
layer,  composed  chiefly  of  firm,  dense  connective 
tissue,  which  is  continuous  with  the  muscular  apo- 
neuroses surrounding  the  bone  ;  and  an  inner  layer, 
which  is  looser  in  texture,  more  vascular,  and  abun- 
dantly furnished  with  variously  shaped  cells.  The 
periosteum  is  attached  to  the  bone  by  connective- 
tissue  fibres,  which  pass  from  the  former  into  the 
substance  of  the  latter,  the  attachment  being  firmer 
at  some  points  than  at  others,  as,  for  example,  near 
the  extremities  of  the  long  bones  and  at  the  points 
of  insertion  of  the  tendons  and  ligaments.  Blood- 
vessels  pass    also     from    the    periosteum    into    the 

jDone. 

3.  Marrozv. — Marrow-tissue  is  found  in  the  cen- 
tral or  medullary  cavity  of  bones,  in  the  tiny  cham- 
bers of  spongy  bone,  and  in  the  Haversian  canals. 
Sometimes  it  has  a  yellowish  color  and  is  fatty, 
sometimes   it  presents   itself  in  the   form  of  a  red- 


66  NORMAL  HISTOLOGY. 

dish  pulp.  Red  marrow  is  found  in  embryos  and 
in  young  animals,  and  in  adults  in  certain  small 
bones  and  in  vertebrae.  In  certain  animals,  such  as 
the  rabbit  and  guinea-pig,  red  marrow  is  found  in 
most  of  the  bones,  even  in  adult  life.  In  adult 
man,  under  normal  conditions,  the  marrow — except 
i  in  the  vertebras,  ribs,  and  certain  small  bones — is  yel- 
low. Yellow  marrow  differs  from  the  red  in  that  it 
contains  a  large  amount  of  fat,  sometimes  consist- 
ing almost  exclusively  of  fat-cells. 

We  find  in  red  marrow,  which  is  best  adapted  for 
study,  blood-vessels  and  spindle-shaped  or  branching 
cells,  which  constitute  the  supporting  framework  of 
the  tissue.  In  the  interstices  of  the  latter  lie  sev- 
eral distinct  kinds  of  cells:  I.  Fat-cells;  2.  smaller 
and  larger  spheroidal  cells,  having  essentially  the 
same  structure  and  character  as  the  lymph-cells,  and 
called,  par  excellence,  marrow-cells;  3.  cells  some- 
what larger  than  the  last  mentioned,  with,  usually,  a 
single  very  irregular-shaped  and  sharply  defined  nu- 
cleus ;  4.  very  large  granular  cells,  which  usually  have 
several  nuclei  scattered  through  the  cell-body,  or 
grouped  on  one  side,  the  so-called  myeloplaxcs  or 
giant  cells.  It  is  not  improbable  that  the  two  last 
varieties  are  only  modified  forms  of  the  same  kind 
of  cells.  In  the  marrow  of  developing  bone  are  seen 
spheroidal,  or  irregularly  cuboidal,  large  granular 
cells,  with  commonly  oval  nuclei,  usually  situated 
at  one  side  of  the  cell-body.    These  are  the  so-called 


BONE.  67 

osteoblasts,  with  which  we  shall  become  better  ac- 
quainted when  we  stud}-  the  process  of  bone-de- 
velopment. 

In  addition  to  the  above  cell-forms,  red  blood- 
cells,  escaped  from  the  blood-vessels,  are  usually 
seen  in  abundance.  Not  infrequently,  when  fresh 
marrow  is  studied,  cells  are  seen  which  in  many 
respects  are  like  the  true  marrow-cells,  but  which, 
with  a  distinct  nucleus,  have  a  homogeneous  cell- 
body  resembling  in  its  color  the  red  blood-cells. 
These  cells  are  the  so-called  nucleated  red  blood-cells, 
and  are  believed  by  some  observers  to  be  destined 
to  lose  their  nuclei,  and  assume  the  character  of  the 
ordinary  red  blood-cells.  Those  who  advocate  this 
view  regard  the  marrow  of  bones  as  one  of  the 
blood-producing  tissues  of  the  body.  The  trans- 
formation of  these  cells  into  red  blood-cells  has 
never  been  directly  observed,  and  as  the  peculiar 
appearance  which  they  present  can  be  accounted  for 
on  other  grounds,  the  formation  of  red  blood-cells 
in  the  marrow,  while  not  improbable,  cannot  yet  be 
regarded  as  definitely  demonstrated. 

PRACTICAL   STUDY. 

Decalcified  Bone. — To  obtain  a  general  view  of  the  struc- 
ture of  bone,  we  have  recourse  to  transverse  and  longitudi- 
nal sections  of  one  of  the  long  bones  (from  man  or  the 
lower  animals,  such  as  the  rabbit  or  dog),  which  has  been 
freed  from  its  lime  salts  by  soaking  in  dilute  acids,  and 
rendered  so  soft  as  to  be  readilv  cut  with  a  razor. 


68  NORMAL  HISTOLOGY. 

Although  various  acids,  such  as  nitric  and  hydro- 
chloric, effect  the  decalcification  of  bone,  solutions  of 
chromic  or  picric  acids  are  preferable,  because,  while 
very  perfectly  removing  the  lime  salts,  they  harden  and 
preserve  the  soft  structures  in  a  most  satisfactory,  manner. 
As  the  salts  of  lime,  as  they  exist  in  bone,  do  not  undergo 
rapid  solution  in  these  acids,  the  bits  of  bone  which  are 
to  be  decalcified  should  be  small,  or  the  process  will  be  a 
very  protracted  one.  They  should  not,  at  most,  be  larger 
than  a  cubic  centimetre.  The  quantity  of  fluid  should 
also  be  quite  large  (two  hundred  to  three  hundred  cubic 
centimetres  to  a  bit  of  bone  of  the  above  size).  Picric 
acid,  although  slow  in  its  action,  is,  on  the  whole,  to  be 
preferred,  because  the  chromic  acid  often  leaves  the  tis- 
sues in  a  granular  or  cloudy  condition,  which  interferes 
with  subsequent  study.  If  chromic  acid  be  employed, 
the  bit  of  bone  is  put  first  into  a  weak  aqueous  solution  of 
one  in  six  hundred  ;  in  a  couple  of  days  it  is  transferred 
to  a  fresh  solution  of  one  in  four  hundred,  and  again  in  a 
couple  of  days  to  another  solution  of  one  in  two  hun- 
dred. A  stronger  solution  than  this  should  not  be  used, 
but  this  should  be  renewed  every  few  days,  and  the  bottle 
frequently  shaken.  In  two  or  three  weeks  the  process 
will  probably  be  completed  ;  this  can  be  ascertained  by 
passing  a  fine  needle  into  the  preparation.  If  it  be  de- 
sirable to  hasten  the  process,  after  a  week  or  ten  days' 
soaking  in  chromic  acid,  as  directed,  a  little  nitric  acid 
may  be  added  to  the  solution  (one  c.c.  to  one  hundred 
c.c).  The  previous  action  of  the  chromic  acid  will  pre- 
vent the  swelling  and  partial  destruction  of  the  soft 
Darts,  which  nitric  acid  alone  causes.     If  picric   acid   be 


BONE.  69 

employed,  a  saturated  aqueous  solution  should  be  used, 
the  preparation  frequently  shaken,  and  additional  crys- 
tals of  the  acid  occasionally  added. 

When  the  bone  has  become  sufficiently  soft  by  either 
of  these  methods,  it  is  allowed  to  soak  for  a  day  in 
water  to  remove  the  excess  of  acid,  and  then  hardened 
and  preserved  in  alcohol.  Longitudinal  and  transverse 
sections  should  be  made,  and,  if  decalcified  by  chro- 
mic acid,  are  best  stained  double  with  hsematoxylin 
and  eosin  ;  if  by  picric  acid,  the  structure  shows  very 
well  after  staining  with  picro-carmine.  Both  may  be 
mounted  in  glycerin.  If  the  periosteum  has  not  been 
removed,  its  structure  and  relation  to  the  bone  are  well 
shown. 

Sections  of  Hard  Bone. — In  such  preparations  as  the 
above,  which  are  mounted  in  fluids,  the  canaliculi  are 
for  the  most  part  invisible,  because  the  fluids  which  fill 
them  possess  very  nearly  the  same  refractive  power  as 
the  basement  substance.  The  cell-spaces  and  canaliculi 
are  best  studied  in  thin  sections  of  hard  bone,  which 
have  been  macerated  for  some  time,  so  as  to  remove  the 
medullary  fat  and  other  soft  parts,  and  then  dried. 
Transverse  or  longitudinal  sections  may  be  made,  the 
latter  being  most  easily  prepared,  because  sections  in 
this  direction  are  less  brittle. 

A  small  piece,  as  thin  as  possible,  should  be  sawn  from 
the  bone  (the  diaphysis  of  a  human  long  bone  answers 
very  well)  in  the  proper  direction  ;  this  is  ground  down 
very  thin  on  a  whetstone  or  grindstone,  or  on  a  plate  of 
glass  with  emery  powder,  the  section  being  held  down 
with  the  ball  of  the  finger  or  a  bit  of  soft  cork.     When  it 


/O  XORMAL  HISTOLOGY. 

has  become  quite  thin,  so  as  to  be  almost  transparent, 
it  is  polished  on  a  dry  oil-stone  free  from  grease,  and 
then  carefully  washed  and  brushed  under  water  with  a 
fine  pencil  to  remove  particles  of  dirt.  It  is  now  al- 
lowed to  dry,  and  is  mounted  in  balsam.  For  this  pur- 
pose the  semi-fluid  Canada  balsam,  such  as  is  used  for 
ordinary  mounting,  should  not  be  employed,  because  it 
would  run  into  the  lacunae  and  canaliculi,  and  render 
them  invisible,  A  bit  of  quite  hard  and  solid  balsam 
should  be  placed  on  a  slide  and  heated  until  it  melts  ; 
just  as  it  begins  to  fairly  cool,  but  before  it  gets  at  all 
hard,  the  slip  of  bone  is  quickly  immersed  in  the  drop 
and  covered.  If  the  proper  moment  has  been  chosen, 
when  the  balsam  is  neither  too  hot  nor  too  cold,  the 
lacunae  and  canaliculi  are  clearly  defined  by  reason  of 
the  air  with  which  they  are  filled.  Usually,  however,  in 
the  most  successful  preparations,  in  the  very  thin  parts  or 
at  the  edges,  part  of  the  canaliculi  have  become  filled 
with  the  balsam  and  rendered  invisible. 

Marrow. — A  long  bone,  from  a  rabbit  or  from  a  child, 
should  be  broken  across  and  a  little  of  the  red  marrow 
scooped  out  and  laid  for  twenty-four  hours  in  a  mixture 
of  alcohol  and  water  (one  to  two).  Fragments  are  then 
teased  on  a  slide  in  glycerin  which  has  been  colored 
with  eosin,  and  mounted  in  the  same.  By  this  process 
the  red  blood-cells  are  destroyed,  and  the  characters  of 
the  so-called  nucleated  red  blood-cells  changed.  For 
the  study  of  the  latter,  fresh  marrow  should  be  teased  on 
a  slide  in  the  indifferent  salt  solution. 


DEVELOPMENT  OF  BONE.  7' 

DEVELOPMENT    OF    BONE. 

At  a  certain  period  of  embryonic  life  no  bone-tis- 
sue is  found  in  the  body,  the  parts  where  it  is  finally 
to  be,  being  occupied  cither  by  cartilage  or  fibrillar 
connective  tissue.  Out  of  these  tissues  the  bone  is 
developed  by  a  process  which,  though  presenting 
considerable  differences  in  detail  in  various  parts  of 
the  body,  is  yet,  in  its  essential  nature,  the  same  in 
all.  We  recognize  three  ways  in  which  bone  is  de- 
veloped :  I.  In  the  substance  of  preexisting  car- 
tilage— intra-cartilaginous ;  2.  beneath  the  perios- 
teum— sub-pcriostcal ;  3.  in  the  substance  of  preex- 
isting fibrillar  connective-tissue  membranes — intra- 
membranous.  In  all  of  these  modes  of  bone-forma- 
tion the  new  bone  seems  to  be  deposited  under  the 
influence  of  certain  large,  granular,  usually  spheroidal 
or  cuboidal  cells,  called  osteoblasts. 

1.  When  bone  is  formed  from  cartilage,  the  latter 
bears  a  general  resemblance  in  shape  to  the  finished 
bone.  The  first  change  which  we  notice  in  such  a 
cartilage  which  is  about  to  undergo  ossification,  is 
that  at  a  certain  point — if  it  be  a  long  bone,  at 
about  the  middle  of  the  diaphysis— the  cartilage-cells 
begin  to  enlarge,  the  basement  substance  between 
them  becoming  partially  absorbed,  and  what  re- 
mains of  the  latter  becoming  infiltrated  with  fine 
granules  of  lime  salts.  Around  this  calcified  por- 
tion we  find  the  blood-vessels  of  the  perichondrium* 


72  NORMAL  HISTOLOGY. 

accompanied  by  marrow-tisssue  and  the  above- 
mentioned  osteoblasts,  growing  into  the  calcified 
cartilage,  absorbing  the  latter  as  they  go,  and  form- 
ing irregular  channels  or  cavities  called  medullary 
spaces.  These  channels  are  first  separated  from 
one  another  by  narrow,  irregular  septa  of  the  cartil- 
age basement  substance  which  remains  unabsorbed, 
and  are  lined  by  layers  of  osteoblasts,  by  whose 
agency  the  septa  become  covered  with  new  bone, 
in  a  manner  presently  to  be  described. 

The  region  in  which  this  new  bone  is  first  depos- 
ited is  called  the  ossification  zone.  As  the  blood- 
vessels, accompanied  by  the  osteoblasts  and  mar- 
row-tissue, proceed  further  and  further  into  the  car- 
tilage, channelling  out  the  medullary  spaces  as  they 
go,  we  always  find — just  in  advance  of  the  ends 
of  the  blood-vessels  and  the  extremity  of  the 
spaces  in  which  they  lie — a  zone  of  calcified  cartil- 
age ;  and  beyond  this,  cartilage-tissue  which  appar- 
ently prepares  the  way  for  the  advancing  marrow- 
spaces  and  newly  forming  bone,  by  very  character- 
istic modifications,  chiefly  in  the  form  and  arrange- 
ment of  its  cells. 

If  we  examine  the  cartilage  at  a  considerable 
distance  from  the  line  of  ossification,  we  find  the  or- 
dinary appearance  of  hyaline  cartilage  with  more  or 
less  flattened  cells.  Approaching  now  the  zone  of 
ossification,  we  find  that  the  cells  are  larger,  are 
arranged  in  rows  or  groups  of  frequently  four,  eight, 


DEVELOPMENT  OF  BONE.  73 

or  sixteen,  etc.,  the  intercellular  substance  being  less 
in  amount,  corresponding  to  the  increase  in  size  and 
number  of  the  cells.  Further  inward,  we  find  the 
cells  still  more  plainly  arranged  in  rows,  very  large, 
sometimes  globular  or  flattened  against  one  another, 
and  the  basement  substance  reduced  to  quite  thin 
septa,  enclosing  spaces  in  which  the  rows  of  large 
cartilage-cells  lie.  Then  comes  a  narrow  zone,  in 
which  the  septa  of  the  basement  substance  are  filled 
with  fine  granules  of  lime  salts — calcification  zone. 
Here  the  cartilage-cells  have  assumed  a  peculiar 
granular  character.  Finally,  still  nearer  we  find  that 
the  lime  salts  have  disappeared  from  the  septa,  and 
that  the  spaces  which  contained  the  large  granular 
cartilage-cells  have  become  continuous  with  the 
advancing  vascular,  bone-walled  marrow-cavities, 
above  described.  It  is  to  be  distinctly  understood 
that  the  calcification  zone  is  not  bone,  but  only  cal- 
cified cartilage ;  the  true  bone  being  first  formed 
after  this  lime  has  disappeared,  on  the  surface  of  the 
septa  in  which  it  was  temporarily  deposited — for 
what  purpose  we  do  not  know. 

Turning  our  attention  now  to  the  exact  way  in 
which  the  bone  is  formed  under  the  influence  of  the 
osteoblasts,  we  find  that  just  beneath  these  cells,  as 
they  lie  along  the  walls  of  the  new-formed  medul- 
lary spaces,  the  basement  substance  of  true  bone 
begins  to  be  deposited,  at  first  in  the  form  of  a  nar- 
row shell  beneath  each  osteoblast.     These  deposits, 


74  NORMAL  HISTOLOGY. 

which  on  cross-sections  have  a  crescentic  shape,  be- 
come thicker  and  thicker,  rising  up  around  the  cell, 
which  they  finally  enclose — the  enclosed  osteoblast 
becoming,  as  it  would  seem,  a  bone-cell.  This 
process  going  on  around  each  osteoblast,  the  walls 
of  the  medullary  cavities  soon  become  covered  with 
a  layer  of  bone  containing  bone-cells.  New  osteo- 
blasts appear  on  the  walls,  and  in  turn  become  en- 
closed in  a  layer  of  bone,  and  thus  the  lamellar 
arrangement  of  bone-tissue  is  produced.  The  re- 
mains of  cartilage  basement  substance  between  the 
medullary  space  thus  covered  by  bone  finally  dis- 
appear in  a  manner  unknown  to  us. 

2.  Hand  in  hand  with  the  formation  of  bone 
within, the  cartilage,  new  bone  is  formed  on  its  sur- 
face beneath  the  perichondrium,  which  thus  becomes 
periosteum.  The  process  of  sub-periosteal  ossifica- 
tion, by  which  the  bone  increases  in  thickness,  is 
dependent  also  upon  the  presence  of  osteoblasts. 
We  find  these  arranging  themselves  along  the  blood- 
vessels which  enter  the  bone,  and  along  the  inner 
layer  of  connective-tissue  fibres  of  the  periosteum, 
and  bone  is  formed  around  them  in  the  manner 
above  described.  New  bone  thus  formed  at  the  sur- 
face appears  at  first  by  no  means  in  the  form  of 
smooth,  continuous  layers,  for  as  the  blood-vessels 
and  connective-tissue  bundles,  along  which  the  oste- 
oblasts lie,  are  arranged  at  varying  angles  with  the 
surface  of  the  bone   and  with  each  other,  the  effect 


DE  I  '/■  LOPM Eh '  1 '  OF  B  ONE.  7  5 

is  to  produce  irregular-branching  cavities,  upon 
whose  walls  the  new  layers  of  bone  are  deposited. 
When  these  branching  cavities  become  filled,  with 
the  exception  of  the  space  occupied  by  the  blood- 
vessels and  marrow-tissue,  by  successive  lamellae  of 
bone,  they  constitute  the  structures  with  which  we 
are  already  familiar  under  the  name  of  Haversian 
canals  and  Haversian  lamellae.  Where  the  forma- 
tion of  bone  has  taken  place  along  the  bundles  of 
connective  tissue,  these  bundles  sometimes  persist 
for  a  long  time,  in  a  modified  form,  among  the 
lamellae,  and  constitute  the  above-mentioned  SJiar- 
peys  fibres. 

Thus,  by  the  transformation  of  cartilage  and  ap- 
position at  the  surface,  the  long  bones  are  formed. 
In  these  bones  the  ossification  progresses  toward  the 
epiphyses,  where  independent  centres  of  ossification 
are  established.  The  lines  of  ossification  approach 
each  other,  and  finally;  when  the  process  of  growth 
in  the  bone  is  complete,  the  band  of  cartilage  which 
separated  them  disappears,  and  epiphysis  and  dia- 
physis  join  to  form  a  single  bone.  As  the  bone 
grows  by  apposition  beneath  the  periosteum,  the 
osseous  tissue  which  was  first  formed  in  the  diaphy- 
sis  is  absorbed,  and  the  medullary  cavity  is  formed 
in  the  place  which  it  originally  occupied. 

3.  When  bone  is  formed  in  membranes  of  fibrillar 
connective  tissue,  as  in  the  skull-cap,  we  notice,  first, 
that  some  of  the  interlacing  bundles  which  occupy 


76  NORMAL  HISTOLOGY. 

the  place  of  the  future  bone  become  infiltrated  with 
lime  salts;  along  these  calcareous  bundles  cells  be- 
come very  abundant,  osteoblasts  appear  and  arrange 
themselves,  and  bone  forms  around  them  just  as  in 
the  other  varieties  of  bone-formation.  Blood-vessels 
and  marrow-tissue  lie  between  the  new-formed  layers 
of  bone,  so  that  at  a  certain  period  of  embryonic  life 
the  bones  of  the  skull-cap  consist  of  a  series  of  bony 
lamellae,  arranged  so  as  to  enclose  branching  and 
communicating  cavities,  which  are  occupied  by 
blood-vessels  and  marrow-tissue,  and  whose  walls  are 
lined  with  osteoblasts.  A  well-defined  periosteum 
is  finally  formed,  beneath  which  successive  layers  of 
new  osseous  tissue  are  deposited,  and  thus  the  bone 
increases  in  thickness  and  acquires  its  smooth  sur- 
faces. 

The  mode  of  origin  of  the  osteoblasts  is  still  very 
obscure.  Many  investigators  believe  that  in  the 
intra-cartilaginous  ossification  they  are  the  large  car- 
tilage-cells which  we  see  at  the  calcification  line, 
which  in  contact  with  the  blood-vessels  become  so 
modified  in  form  and  function  as  to  assume  the  role 
of  bone-formers.  Others  assert  that  the  large  carti- 
lage-cells disintegrate  and  disappear,  and  that  the 
osteoblasts  are  produced  from  cells  which  accom- 
pany the  blood-vessels.  Still  others  regard  them  as 
white  blood-cells,  modified  and  endowed  with  new 
functional  powers.  In  the  intra-membranous  and 
sub-periosteal  ossification,  many  suppose  that  they 


DE  VELOPMENT  OF  BONE.  JJ 

are  formed  from  connective-tissue  cells.  So  little  is 
absolutely  known,  however,  as  to  their  genesis,  that 
while  recognizing  their  importance  in  bone-forma- 
tion, we  can  regard  none  of  these  various  theories  as 
to  their  origin  as  definitely  established. 

PRACTICAL    STUDY. 

Intra-cartilaginons  and  Sub-periostcal  Ossification. — A 
long  bone,  from  a  nearly  mature  foetus  or  a  young  ani- 
mal, should  be  carefully  removed  without  injuring  the 
periosteum,  and  decalcified.  Thin  longitudinal  sections 
are  made  with  a  razor  through  the  ossification  zone,  em- 
bracing the  tissue  for  a  considerable  distance  on  either 
side  of  it.  The  sections  are  stained  double  and  mounted 
in  balsam. 

Iiitra-nicmbranous  Ossification. — To  study  the  early 
stages  of  this  process,  a  young  embryo  (if  from  the  sheep 
or  pig,  four  to  six  cms.  long)  should  be  soaked  for  a  few 
days  in  Muller's  fluid,  and  a  bit  corresponding  to  the  por- 
tion of  one  of  the  parietal  bones  cut  out,  and  the  skin, 
muscles,  and  dura  mater  torn  away  with  forceps  under 
water.  The  membrane  in  which  ossification  is  occurring 
is  now  to  be  carefully  brushed  with  a  stiff  pencil  until 
it  is  thin  enough  to  be  examined  with  tolerably  high 
powers.     It  is  stained  double  and  mounted  in  balsam. 

The  irregular  chambers,  lined  with  osteoblasts  and 
filled  with  blood-vessels  and  marrow,  which  are  formed 
in  the  bones  of  the  skull-cap  at  a  later  period,  are  well 
shown  by  transverse  sections  through  the  decalcified 
skull-bones  of  an  older  foetus  (human,  at  about  six  or 


?S  NORMAL  HISTOLOGY. 

seven  months,  or  from  the  beef  or  sheep,  sixteen  to 
twenty  cms.  long).  These  should  be  stained  and  mounted 
as  above. 

TEETH. 

The  teeth  have  many  structural  features  in  com- 
mon with  bone.  The  chief  bulk  of  the  tooth  is 
made  up  of  homogeneous,  brittle  basement  sub- 
stance much  harder  than  bone,  called  dentine.  The 
dentine  contains  lime  salts,  and  is  permeated  by  a 
multitude  of  fine  branching  channels  which  radiate 
from  a  central  cavity,  called  the  pulp  cavity,  which 
the  dentine  encloses.  These  delicate  channels  in 
the  dentine  are  analogous  with  the  canaliculi  of 
bone. 

The  pulp  cavity  is  filled  with  a  soft  vascular 
tissue,  called///^,  containing  irregular-shaped,  often 
branching  cells,  and  nerves.  Along  the  sides  of  the 
pulp  cavity  lie  spheroidal  or  ovoid  cells,  which  send 
off  branches  into  the  pulp  and  also  into  the  above- 
mentioned  delicate  channels  in  the  dentine.  These 
cells  are  called  odontoblasts  or  dentine  cells,  and  are 
usually  considered  to  be  the  analogues  of  the  bone- 
cells.  The  pulp  cavity  is  open  at  the  root  of  the 
tooth  for  the  admission  of  vessels  and  nerves.  Sur- 
rounding the  root  of  the  tooth  is  a  thin  layer  of 
bone  called  cement.  At  the  crown  of  the  tooth  th^ 
dentine  is  completely  covered  by  a  layer,  of  vary- 
ing thickness,  of  an  extremely  hard  substance  called 


TEE  TH.  79 

enamel.  The  enamel  consists  of  a  series  of  closely 
packed,  small,  wavy  or  undulating  prisms,  placed 
edgewise  upon  the  surface  of  the  dentine,  and  cov- 
ered, over  the  free  surface  of  the  tooth,  by  a  hard, 
tough,  structureless  membrane,  called  the  enamel 
cuticle. 

PRACTICAL    STUDY. 

Hard  Teeth. — To  study  the  hard  parts  of  teeth,  thin 
sections  of  a  macerated  and  dried  tooth  should  be  ground 
down  by  the  method  described  when  we  were  studying 
hard  bone,  and  mounted,  with  the  same  precautions,  in 
hard  Canada  balsam. 

Decalcified  Teeth. — The  soft  parts  of  teeth  may  Le 
studied  in  sections  from  teeth  which  have  been  decalci- 
fied with  picric  acid.  The  tooth  should  be  broken 
across,  so  as  to  expose  the  pulp  cavity  and  hasten  the 
action  of  the  solvent.  Sections  are  stained  with  hema- 
toxylin and  eosin,  and  mounted  in  glycerin. 


CHAPTER  V. 

BLOOD  AND  LYMPH. 

BLOOD. 

ALTHOUGH  strikingly  different  in  physical  char- 
acter from  most  animal  tissues,  we  must  yet  regard 
blood  and  lymph  as  true  tissues — tissues  with  a  fluid 
intercellular  substance.  Let  us  first  consider  the 
blood. 

In  normal  human  blood  we  find  suspended  in  a 
colorless  fluid,  the  plasma,  three  distinct  kinds  of 
formed  elements:  I.  Colorless  blood-cells  ;  2.  red  blood- 
cells  ;  3.  variously  shaped  free  granules. 

1.  Colorless  or  White  Blood-cells  or  Leucocytes. — 
These  are  small,  usually  spheroidal  nucleated  cells, 
without  a  membrane,  the  cell-body  being  finely,  or 
sometimes  coarsely,  granular.  The  nuclei,  of  which 
there  may  be  one  or  more,  are  not  usually  visible  in 
the  living  cells  on  account  of  the  granular  character 
of  the  body  which  conceals  them,  and  are  of  vary- 
ing form, —  sometimes  spheroidal  or  dumb-bell- 
shaped,  sometimes  having  the  form  of  a  bent  or 
twisted  cylinder,  and  again  entirely  irregular. 

These  cells  possess  the  power,  under  favorable 
conditions,  of  spontaneous  movement.    They  change 


BLOOD.  8 1 

their  form  and  place.  While,  when  in  a  state  of 
rest,  they  assume  in  general  the  spheroidal  form,  as 
above  stated,  we  find  that  when  they  become  active 
they  send  out  variously  shaped  processes,  some  fine 
and  delicate,  others  broad  and  of  very  irregular 
shape.  We  often  see,  after  a  process  has  been 
thrown  out,  that  it  becomes  gradually  larger  and 
larger,  the  cell-body  becoming  correspondingly 
smaller,  until  finally  the  whole  cell  seems  to  have 
passed  over  into  the  process,  thus  moving  forward. 
Sometimes  processes  are  thrown  out  and  again  with- 
drawn, and  not  infrequently  the  whole  cell  flattens 
out  into  an  irregular-shaped  mass,  so  thin  as  to  be 
almost  invisible.  Not  infrequently  clear  rounded 
spaces,  called  vacuoles,  suddenly  appear  in  the  cell- 
body  during  its  movements,  and  either  remain  for 
some  time,  or  soon  disappear  as  suddenly  as  they 
came. 

These  movements  are  called  amoeboid  movements ; 
they  are  always  very  slow,  and  are  greatly  influenced 
by  the  temperature,  density,  and  oxygen-content  of 
the  fluid  in  which  they  lie.  By  virtue  of  this  loco- 
motive power  the  white  blood-cells  perform  certain 
evolutions  within  the  vessels ;  they  escape  through 
their  walls,  and  sometimes  singly,  sometimes  in  vast 
numbers,  move  through  the  tissues  in  the  larger  and 
smaller  lymph-spaces.  This  emigration  of  white 
blood-cells  occurs  apparently,  to  a  slight  extent, 
under  normal  conditions:  but  it  is  under  patho- 
logical conditions  that  it  is  most  active. 


82  NORMAL  HISTOLOGY. 

The  nuclei  of  these  cells  become  visible  on  the 
application  of  a  variety  of  agents  which  determine 
the  death  of  the  cells,  such  as  acetic  acid,  dilute 
alcohol,  and  certain  coloring  agents.  The  cells  vary 
greatly  in  size,  but  on  the  average  are,  in  man, 
larger  than  the  red  cells. 

2.  Red  Blood-cells. — These  cells,  having  in  man  the 
form  of  bi-concave  discs,  consist  apparently,  simply 
of  a  cell-body  without  membrane  or  nucleus.  Al- 
though when  crowded  together  in  great  numbers 
they  give  the  blood  a  distinct  red  color,  when  seen 
singly  they  have  a  greenish-yellow  tint.  The  cell- 
body  is  very  soft  and  pliable,  jelly-like,  changing  its 
shape  on  the  slightest  pressure.  It  is  more  deeply 
colored  at  the  periphery,  when  seen  from  the  side, 
because  of  its  greater  thickness  at  that  part.  Owing 
to  the  peculiar  shape  of  the  cell,  it  acts  as  a  lens 
upon  the  light  passing  through  it,  and  its  central 
portion  is  either  light  or  dark,  depending  upon 
whether  the  objective  is  approached  to  or  with- 
drawn from  it.  When  examined  fresh  in  the  plasma, 
many  of  the  cells  arrange  themselves  closely  to- 
gether side  by  side,  in  longer  and  shorter  rows. 

If  water  is  mixed  with  blood,  the  red  cells  soon 
begin  to  swell  and  lose  their  color.  Sometimes 
one  side  swells  faster  than  the  other  and  the  cells 
become  cup-shaped  ;  finally  they  become  globular, 
are  considerably  larger  than  at  first,  and  colorless. 

On  being  drawn   from   the  vessels,  if  the  plasma 


BLOOD.  S3 

be  allowed  to  evaporate,  or  if  certain  fluids — such  as 
strong  solutions  of  common  salt  or  bichromate  of 
potassium — be  added,  a  part  of  the  red  blood-cells 
lose  their  regular  shape,  their  edges  become  crenu- 
late  and  jagged,  and  they  sometimes  seem  to  be- 
come smaller;  finally,  they  assume  the  form  of  ir- 
regular globular  masses,  beset  with  short,  blunt 
spines.  Various  other  irregular  forms  arc  produced 
under  the  same  circumstances  which  it  is  not  neces- 
sary to  describe  here.  The  addition  of  water  causes 
the  spines  to  disappear,  and  the  cells  swell  up  and 
assume  a  globular  form.  These  changes  in  form, 
produced  by  chemical  agents  and  by  change  in  the 
density  of  the  fluid  in  which  the  cells  lie,  should  not 
be  mistaken  for  an  expression  of  vitality,  or  re- 
garded as  analogous  with  the  amoeboid  movements 
of  the  white  blood-cells. 

The  red  blood-cells  are  much  more  abundant  than 
the  white,  there  being  in  normal  human  blood  about 
350  to  500  of  the  former  to  one  of  the  latter.  It  is 
estimated  that  in  man  there  are  between  four  and 
five  million  red  blood-cells  in  one  cubic  millimetre 
of  blood.  The  diameter  of  the  average  cell  is 
about  xxg-th  of  a  millimetre,  or  about  7.9  j.1.1  Not 
infrequently  in  normal  blood — very  often  under  pa- 
thological conditions — red  blood-cells  are  seen  which 

1  The  Greek  letter  /*  is  frequently  employed  to  represent  the 
micro-millimetre,  or  the  one-thousandth  part  of  a  millimetre,  this 
having  been  widely  adopted  as  the  unit  for  microscopic  measurement. 
In  English  measurement  the  average  red  blood-cell  has  a  diameter  of 
about  -32Vfftn  °f  an  inch. 


84  NORMAL  HISTOLOGY. 

are  much  smaller  than  the  above-described  forms, 
and  are  often  spheroidal  in  shape. 

The  red  blood-cells  ov.  e  their  color,  as  well  as 
their  capacity  for  performing  certain  important 
physiological  functions,  to  the  presence  in  them  of 
a  crystallizable  substance  called  licsmoglobin.  The 
exact  relation  existing  between  the  haemoglobin  and 
the  substance  of  the  cell  is  but  little  understood. 
They  are  but  loosely  combined,  for  the  haemoglobin 
is  readily  dissolved  out  by  water,  which  itself  be- 
comes colored,  while  the  colorless  and  swollen  cell 
or  stroma  is  left  behind.  The  shape  of  haemoglobin 
crystals  obtained  from  the  blood  of  different  ani- 
mals is  not  always  the  same  ;  those  from  human 
blood  have,  in  general,  the  form  of  rhombic  prisms. 

3.  Free  Granules. — These  are  always  found  in 
greater  or  less  number  in  human  blood  when  drawn 
from  the  vessels.  They  are  chiefly  of  two  kinds ; 
either  small  spherical  bodies,  somewhat  resembling 
fat-droplets,  or  variously  shaped,  often  angular 
masses,  looking  not  unlike  fragments  of  white  blood- 
cells.  They  are  sometimes  seen  singly,  sometimes 
in  clusters.     Their  nature  is  still  undetermined. 

The  above  general  description  of  the  blood-cells 
of  man  applies,  with  few  exceptions,  to  other  mam- 
malia. The  differences  in  size,  however,  which  ex- 
ist between  the  red  blood-cells  of  man  and  those  of 
certain  of  the  mammalia,  are  so  considerable  that 
they  can  be  distinguished  with  certainty  from  one 


BLOOD.  85 

another  by  microscopic  measurements.  It  should, 
on  the  other  hand,  never  be  forgotten  that  the  red 
blood-cells  of  certain  other  mammalia,  c.  g.t  the  dog, 
have  so  nearly  the  same  average  diameter  as  those 
of  man  that  they  cannot  be  distinguished  with  ab- 
solute certainty  by  measurements.  In  other  verte- 
brates the  form  and  character  of  the  red  blood-cells 
differ  greatly  from  those  above  described  ;  being  for 
the  most  part  oval,  and  having  a  distinct  nucleus. 

The  intercellular  substance  or  plasma  of  freshly 
drawn  blood  is  perfectly  homogeneous  ;  but  if  we 
allow  it  to  coagulate,  we  find,  on  subjecting  it  to 
microscopical  examination,  that  in  addition  to  the 
above-described  elements,  a  multitude  of  delicate 
filaments  lie  among  the  cells,  stretching  in  all  direc- 
tions, and  joining  each  other  at  frequent  interval-, 
forming  an  irregular-meshed  net.  We  find,  more- 
over, if  we  examine  a  clot  from  which  the  red  cells'" 
have  been  carefully  removed  or  rendered  invisible, 
that  in  many  places  the  filaments  are  grouped  around 
irregular-shaped  granules,  looking  like  those  above 
described  in  normal  blood  ;  and  if  the  process  of 
coagulation  be  carefully  observed,  they  may  be  seen 
to  actually  shoot  out  from  these  granules,  which 
thus  seem  to  form  starting-points  for  their  forma- 
tion. The  substance  which  thus  separates  from  the 
plasma  is  called  fibrin. 


86  NORMAL  HISTOLOGY. 

LYMPH. 

In  lymph,  in  addition  to  the  plasma  from  which 
fibrin  is  formed  on  separation  from  the  body,  we 
find  spheroidal  cells  identical  in  structure  with  the 
white  blood-cells  ;  sometimes  a  few  red  blood-cells; 
and  variously  shaped  granules  or  minute  globules, 
composed  apparently  of  a  combination  of  albu- 
minoid material,  with  fat.  These  globules,  in  that 
variety  of  lymph  called  chyle,  are  so  abundant  as  to 
give  the  fluid  a  milky  appearance. 

Origin  of  Blood-cells — Direct  observation  has 
shown  that,  in  some  animals  at  least,  the  white 
blood-cells  can  multiply  by  division.  Whether  the 
cells  which  supply  the  place  of  those  which  seem 
to  be  used  up  in  the  process  of  growth  and  repara- 
tion are  produced  in  this  way,  and  if  so,  whether 
the  division  occurs  in  the  blood-  or  lymph-vessels, 
or  in  the  cell-spaces  of  the  connective  tissue,  or  in 
certain  special  organs,  or  whether  they  are  pro- 
duced in  a  manner  entirely  unknown  to  us — these 
are  questions  not  only  of  theoretical  but  of  practi- 
cal interest ;  but,  in  spite  of  much  research,  and  the 
accumulation  of  many  observations  bearing  on  the 
matter,  we  are  still  unable  to  give  them  a  definite 
answer.  Still  more  obscure,  if  possible,  is  the  origin 
of  the  red  blood-cells.  Although  in  the  adult  man  they 
seem  to  possess  no  nucleus,  yet  in  embryonic  life 
they  certainly  are  furnished  with  that  structure;  we 


L  YMPIl.  87 

find  nucleated  red  blood-cells.  Now,  it  has  been  re- 
cently shown  that  in  certain  parts  of  the  body,  in 
adult  life,  cells  occur  which  in  many  respects  resem- 
ble the  nucleated  red  blood-cells  of  the  embryo ; 
such  cells  are  found,  for  example,  in  the  spleen,  in 
the  red  marrow  of  bones,  etc. 

The  most  plausible  theory  in  regard  to  the  matter 
is,  that  in  certain  parts  of  the  body — spleen,  mar- 
row, lymph-glands,  and  liver — white  blood-cells  are 
produced,  a  part  of  which  are  changed  into  the  red 
blood-cells.  The  so-called  nucleated  red  blood-cells 
are  supposed  to  be  intermediate  forms.  It  must  be 
remembered,  however,  that  this  view  is  not  estab- 
lished as  yet,  and  many  observers  do  not  ascribe  to 
the  so-called  nucleated  red  blood-cells  the  signifi- 
cance upon  which  the  advocates  of  this  theory 
insist. 

PRACTICAL   STUDY. 

Fresh  Human  Blood. — This  may  be  obtained  by  tying 
a  cord  tightly  around  the  finger  to  cause  congestion,  and 
then  pricking  it  sharply  at  the  side  of  the  nail  with  a 
bright,  clean  sewing  needle.  A  small  drop  is  received  on 
a  slide  and  covered  at  once,  care  being  taken  not  to  press 
upon  the  cover-glass.  The  film  of  blood  should  be 
very  thin  or  the  crowding  of  the  cells  will  interfere  with 
the  observation.  As  the  plasma  evaporates,  the  changes 
in  form  due  to  shrinkage  of  the  red  blood-cells  may  be  ob- 
served near  the  edges  of  the  preparation.  Finally,  a 
drop  of  water  should  be  allowed  to  run  under  the  cover- 


88  NORMAL  HISTOLOGY. 

glass  and  its  action  observed,  first,  in  causing  the  dis- 
appearance of  the  crenulations  on  the  shrunken  cells  and 
the  swelling  of  ail  the  red  blood-cells,  and  second,  in  dis- 
solving the  haemoglobin  out  of  them. 

Crystals  of  Hcemoglobin  from  Bat's  Blood. — Although 
readily  separated  by  water  from  its  combination  with  the 
stroma  of  the  red  blood-cells  of  man,  the  haemoglobin 
does  not  readily  crystallize.  But  when  separated  from 
the  red  cells  of  certain  animals,  the  rat  for  example,  it 
commences  to  crystallize,  under  favorable  conditions,  al- 
most immediately.  A  small  drop  of  rat's  blood  is  mixed 
on  a  slide  with  an  equal  quantity  of  water,  covered  and 
examined  at  once.  The  color  begins  to  be  discharged 
from  the  red  cells  very  soon,  and  within  a  few  moments, 
near  the  edges  of  the  cover-glass,  small  crystals  may  be 
found  in  abundance.  If  the  specimen  is  set  aside  and 
examined  after  a  few  hours,  many  very  large  and  beauti- 
ful prismatic  crystals  may  be  seen.  Haemoglobin  crys- 
tals do  not  keep  long  enough  for  permanent  preserva- 
tion. 

Demonstration  of  the  Nuclei  of  the  White  Blood- cells. 
— In  order  to  see  their  outlines  distinctly,  the  nuclei 
must  be  stained  and  the  cell-body  rendered  transparent. 
This  may  be  accomplished  by  mixing  on  a  slide  a  small 
drop  of  blood  from  the  finger  with  an  equal  quantity  of 
the  following  fluid  : 

Saturated  alcoholic  solution  of  Analine  Red         I  part, 

Alcohol 5  parts, 

Water 10  parts. 

After  thoroughly  mixing  the  blood  with  the  fluid,  and 


L  YMPII.  QQ 

covering,  it  will  be  found  that  while  the  red  cells  are  par- 
tially decolorized  and  inconspicuous,  the  bodies  of  the 
white  blood-cells  have  become  transparent  and  their 
nuclei  are  stained  deep  red. 

Amoeboid  Movements. — These  are  most  conveniently 
studied  in  the  blood  or  lymph  from  one  of  the  cold- 
blooded animals,  such  as  the  frog,  for  they  occur  here  at 
the  ordinary  temperatures  of  the  air,  while  artificial  heat 
must  be  resorted  to  if  we  would  maintain  the  blood  of 
warm-blooded  animals  at  a  proper  temperature  for  their 
occurrence. 

The  leg  and  toes  of  a  frog  having  been  carefully 
cleansed,  the  tip  of  one  of  the  toes  is  snipped  off,  and  on 
stripping  the  leg  downward  with  the  thumb  and  finger, 
a  drop  of  mixed  blood  and  lymph  will  presently  exude 
from  the  toe.  This  is  received  on  a  slide,  protected  from 
pressure  by  a  bit  of  hair,  and  covered.  To  prevent  evap- 
oration of  the  plasma,  a  rim  of  oil  is  painted  around  the 
edge  of  the  cover.  On  focussing  now  upon  the  specimen, 
white  blood-cells  will  readily  be  found,  and  selecting  one 
which,  by  its  irregular  shape,  indicates  its  activity,  the  at- 
tention must  be  fixed  upon  this  cell  and  sketches  of  its 
form  made  at  short  intervals — every  two  minutes.  Al- 
though the  movement  is  usually  too  slow  to  be  actually 
detected  by  the  eye,  if  the  cell  is  fairly  active,  it  will  be 
sufficiently  evident,  after  a  few  sketches,  that  it  has 
changed  its  shape  and  perhaps  its  place  also. 

If  the  temperature  of  the  room  be  low,  the  movements 
may  be  tardy  in  commencing,  and  they  can  be  hastened 
by  holding  the  finger  or  any  warm  object  for  a  moment 
near  the  cover.     It  should  be  borne  in  mind  that  in  the 


90  NORMAL  HISTOLOGY. 

frog's  blood  the  red  cells  are  oval  and  nucleated,  and  that 
they  are  also  larger  than  the  colorless  cells. 

Fibrin. — A  small  quantity  of  blood  maybe  whipped  and 
the  cells  washed  from  the  clot  by  a  stream  of  water,  and  a 
fragment  of  the  remaining  substance  teased  in  water  on  a 
slide  and  studied  ;  but  the  objects  thus  obtained  are  not 
altogether  satisfactory,  since  the  relation  of  the  fibrillar  to 
one  another  is  disturbed  and  no  light  is  thrown  on  the 
way  in  which  they  are  formed.  For  the  accomplishment 
of  these  ends,  the  following  method  may  be  employed  :  a 
medium-sized  drop  of  blood  is  received  on  a  slide  and 
immediately  covered  ;  after  a  few  moments,  when  coagu- 
lation has  occurred,  the  cover-glass  is  gently  raised  with 
the  forceps,  and  the  blood-cells  washed  out  of  the  clot  by 
allowing  drops  of  water  to  fall  upon  the  inverted  cover- 
glass  from  a  pipette  held  a  few  inches  above  the  prepara- 
tion. (If,  in  removing  the  cover-glass,  the  clot  adheres 
to  the  slide,  the  water  is,  of  course,  to  be  applied  here.) 
When  no  more  color  is  seen  in  the  clot,  a  drop  of  the  above 
solution  of  Analine  Red  is  placed  upon  it,  and  it  is  again 
covered.  The  fibrin  will  be  seen  in  the  form  of  minute 
inosculating  filaments  which  often  radiate  from  what  ap- 
pear to  be  the  above-described  free  granules. 


CHAPTER  VI. 

MUSCULAR  TISSUE. 

Certain  muscles  are  under  the  control  of  the 
will,  and  are  called  voluntary  muscles ;  and  as  these 
have,  as  we  shall  see  presently,  a  very  characteristic 
transverse  striation  of  their  structural  elements, 
they  are  also  called  striated  voluntary  muscles.  To 
this  class  belong,  among  others,  the  muscles  of  lo- 
comotion and  the  voluntary  muscles  of  the  trunk 
and  head.  Other  muscles  are  not  under  control  of 
the  will,  and  are  hence  called  involuntary.  A  cer- 
tain portion  of  these  involuntary  muscles  possess 
the  same  striation  of  their  elements  as  the  volun- 
tary muscles,  and  hence  are  called  involuntary 
striated  muscles.  These  are  found  in  the  heart.  In 
another  kind  of  involuntary  muscles,  such  as  is  found 
in  the  intestine  and  bladder,  the  elements  do  not 
possess  the  same  kind  of  striations,  and  they  are 
hence  called  smooth  or  non-striated  involuntary  mus- 
cles. 

We  have  thus  to  study  three  kinds  of  muscles  : 

T        ,      ,  ,       (  a,  smooth  or  non-striated, 

1.  Involuntary  muscles  \  7     ,  .  ,    , 

/  0)  striated. 

2.  Voluntary  muscles  (striated). 

91 


g2  NORMAL  HISTOLOGY. 

I.    a. SMOOTH    MUSCULAR    TISSUE. 

Smooth  muscular  tissue  is  made  up  of  very  much 
elongated,  narrow,  pointed,  usually  fusiform  cells. 
These  cells  are  commonly  arranged  in  groups  or  bun- 
dles, enclosed  in  connective  tissue  and  supplied  with 
blood-vessels  and  nerves.  The  cell-body,  although 
usually  fusiform,  is  sometimes  flattened  and  band- 
like, often  divided  at  the  ends  into  two  pointed  ex- 
tremities. Owing  to  pressure  from  adjacent  parts, 
the  fusiform  cell-bodies  are  often  more  or  less  flat- 
tened at  the  sides,  presenting  on  cross-section  an 
irregular  polygonal  contour.  The  cell-body  has  an 
indistinct  longitudinal  striation,  and  frequently  in  the 
vicinity  of  the  nucleus  a  few  shining  granules  are 
seen.  The  nucleus  is  usually  narrow  and  much 
elongated,  rod-like,  and  commonly  encloses  one  or 
more  nucleoli.  It  usually  lies  near  the  middle  of 
the  cell,  which  is  often  thickened  or  bulging  at 
that  point. 

These  cells  lie  side  by  side  or  lap  over  one 
another  at  the  ends,  and  are  joined  together  by  a 
small  amount  of  an  albuminoid  cement  substance. 
These  smooth  muscle-cells  are  variously  grouped  in 
different  parts  of  the  body  ;  sometimes  crowded 
together  in  solid  bundles,  which  are  arranged  in 
layers  and  surrounded  by  connective  tissue,  as  in 
the  intestines  ;  sometimes  arranged  in  narrow  inter- 
lacing   fascicles,    as    in    the    bladder,    or    scattered 


MUSCULAR   TISSUE.  93 

singly  through  certain  tissues  ;  sometimes  wound 
in  single  or  double  layers  around  the  blood-vessels; 
and  again  running  in  various  directions  and  associ- 
ated with  bands  of  connective  tissue,  they  form 
large  compact  masses,  as  in  the  uterus. 

The  longitudinal  striation — which,  under  favorable 
circumstances,  is  seen  on  the  cell-body — is  not  a 
mere  surface  marking,  but  extends  deep  into  the 
cell,  as  may  be  seen  in  transverse  sections  of  suitably 
prepared  cells  where  fine  lines  are  observed  passing 
inward  from  the  periphery  of  the  cell  toward  the 
nucleus.  The  blood-vessels  supplying  this  tissue 
form  for  the  most  part  elongated  net-works  through- 
out its  substance. 

PRACTICAL   STUDY. 

Isolated  Cells. — These  we  obtain  by  teasing  bits  of  the 
tissue,  but  as  they  are  firmly  bound  together  by  the 
cementing  substance,  this  must  first  be  dissolved  or 
softened.  This  can  be  conveniently  accomplished  by 
soaking  a  bit  of  the  tissue — the  wall  of  the  intestine  for 
example — for  forty-eight  hours  in  an  aqueous  solution  of 
bichromate  of  potassium  (1  to  Soo).  Shreds  of  the 
muscular  layer  are  torn  off,  slightly  stained  with  hema- 
toxylin, washed,  and  then  carefully  teased  in  glycerin 
which  has  been  colored  by  eosin.  To  prevent  the  shrink- 
age of  the  cells  under  the  action  of  the  bichromate,  it  is 
well  to  enclose  a  short  segment  of  the  intestine,  dis- 
tended with  the  solution,  between  two  ligatures,  and  im- 
merse it  in  a  vessel  of  the  same  solution. 


94  NORMAL  HISTOLOGY. 

The  cells  may  be  more  perfectly  separated  by  soaking 
a  bit  of  the  fresh  tissue  for  a  few  minutes  in  forty-per- 
cent, solution  of  caustic  potash  and  then  teasing,  but 
they  cannot  be  stained  or  preserved. 

Transverse  and  Longitudinal  Sections  of  the  Cells. — 
The  intestine  of  the  cat  is  well  adapted  for  this  prepara- 
tion, since  here  the  muscle-cells  are  unusually  large.  A  seg- 
ment having  been  distended  as  above  with  Miiller's  fluid, 
it  should  be  immersed  for  ten  days  in  the  same,  then 
carefully  washed  and  put  for  a  day  or  two  in  strong  al- 
cohol. A  bit  is  then  cut  out,  embedded  between  pieces 
of  hardened  liver,  and  thin  sections  made  in  a  direction 
exactly  at  right  angles  to  the  axis  of  the  gut.  The  sec- 
tions are  stained  double  and  mounted  in  balsam  or  glyc- 
erin. In  such  a  preparation  two  layers  of  muscle-cells 
are  seen  :  in  one  the  cells  are  seen  in  transverse,  in  the 
other  in  longitudinal,  section.  Since  the  cells  lap  over 
one  another,  in  the  transverse  sections  the  forms  which 
they  present  will  obviously  differ,  depending  upon 
whether  they  have  been  cut  across  at  the  level  of  the 
nucleus,  or  at  a  point  nearer  the  extremity.  In  such  a 
preparation,  the  cementing  substance  between  these  cells 
may  be  seen  in  the  transverse  sections  ;  and  the  serosa 
and  mucous  membrane  are  seen  on  opposite  sides  of  the 
muscular  layers. 

Muscle-cells  of  Frog's  Bladder. — Elegant  pictures  of 
very  much  elongated,  slender  muscle-cells,  lying  singly  or 
arranged  in  narrow  interlacing  fascicles,  may  be  obtained 
from  the  frog's  bladder  by  the  following  method  :  the 
spinal  cord  of  a  frog  being  broken  up,  the  abdominal 
cavity  is  largely  opened    by    a    crucial    incision,    and    a 


MUSCULAR   TISSUE.  95 

curved  canula,  attached  to  a  small  syringe  filled  with  a 
saturated  solution  of  bichromate  of  potassium,  is  passed 
into  the  cloaca  and  directed  forward  into  the  bladder. 
The  fluid  is  now  slowly  injected,  and  when  the  bladder 
is  partially  distended  a  ligature  is  thrown  around  its 
base,  and  the  injection  continued  till  the  organ  is  fully 
distended.  The  ligature  is  now  drawn  tight  and  the 
canula  withdrawn.  The  bladder  is  cut  out,  still  dis- 
tended, and  put  in  the  same  bichromate  solution,  where 
it  remains  for  three  days,  when  it  is  washed  and  trans- 
ferred to  alcohol.  After  twenty-four  hours  it  may  be 
opened,  and  a  bit  cut  out,  the  epithelium  carefully 
brushed  from  the  inner  surface,  and  stained  double  and 
mounted  in  glycerin.  In  addition  to  the  muscle-cells, 
the  nuclei  of  the  endothelial  cells  covering  the  peritoneal 
surface  will  be  seen,  as  well  as  connective-tissue  cells  and 
fibres  in  the  wall  of  the  bladder. 

2. STRIATED    VOLUNTARY    MUSCULAR    TISSUE. 

As  the  involuntary  striated  or  heart  muscle  oc- 
cupies, in  structure,  an  intermediate  position  between 
the  smooth  and  the  voluntary  striated  muscle,  we 
shall  find  it  advantageous  to  postpone  its  study 
until  we  have  considered  the  other  varieties. 

Voluntary  striated  muscle,  to  which  the  greater 
part  of  the  muscular  tissue  of  the  body  belongs,  is 
made  up  of  narrow,  cylindrical,  cord-like  elements, 
of  varying  length  and  thickness,  called  muscular 
fibres.  These  are  grouped  in  variously  shaped 
bundles    or    fascicles,    surrounded    by    connective- 


96  NORMAL  HISTOLOGY. 

tissue  envelopes  or  sheaths,  and  abundantly  sup- 
plied with  blood-vessels  and  nerves.  Let  us  first 
study  the  structure  of  the  individual  fibres.  They 
consist  of  three  distinct  elements:  1,  contractile 
substance,  forming  the  centre  and  making  up  most 
of  the  bulk  of  the  fibre  ;  2,  nuclei,  which  in  man 
and  most  warm-blooded  animals -lie  scattered  upon 
the  surface  of  the  contractile  substance  ;  3,  the 
sarcolcmwa,  a  thin  homogeneous  sheath  or  tube, 
which   tightly  encloses  the  other  elements. 

1.  If  we  examine  a  fresh  muscle-fibre,  or  one 
which  has  been  hardened  under  favorable  condi- 
tions, with  moderately  high  powers,  we  see  that  the 
contractile  substance  is  indistinctly  longitudinally 
striated  ;  and  if  we  treat  muscle  with  certain  chem- 
ical agents,  such  as  chromic  acid  or  its  salts,  we 
find  that  by  slightly  teasing,  the  fibres  break  up 
along  the  longitudinal  striae  into  a  multitude  of  fine 
fibrillar,  which  are  called  primitive  musclc-fibriilce. 
Asrain,  if  we  examine  the  fresh  or  hardened  fibres 
still  further,  we  find  that  in  addition  to  the  longi- 
tudinal striations,  they  are  crossed  by  more  promi- 
nent, narrow,  alternating,  dark  and  light  bands  or 
stripes,  the  relative  width  of  the  stripes  varying  ac- 
cording as  the  muscle  is  seen  in  a  state  of  contrac- 
tion  or  relaxation.  Still  further,  if  we  soak  a  fresh 
muscle  for  twenty-four  hours  in  a  half-per-cent. 
solution  of  hydrochloric  acid,  and  then  tease  it,  we 
find  that   the   fibres,  instead    of  breaking   up  longi- 


MUSCULAR   TISSUE.  97 

tudinally  into  fibrillar,  break  across  transversely  into 
thin  discs.  We  thus  see  that,  by  breaking  up  in 
these  two  directions,  we  may  conceive  of  the  fibre 
as  being  resolvable  into  a  multitude  of  tiny  pris- 
matic structures,  which  are  called  sarcous  elements. 
The  central  portion  of  each  prism  or  sarcous  ele- 
ment is  occupied  by  a  dark  portion,  while  at  each 
end  is  a  lighter  zone.  The  light  and  dark  zones  of 
the  sarcous  elements,  when  the  latter  are  grouped 
together,  form  the  alternating  light  and  dark  bands 
of  the  fibres.  It  is  believed  by  many  observers 
that  the  sarcous  elements  are  definite  and  independ- 
ent structures,  in  which  the  dark  portion  is  the 
contractile  element,  and  that  they  are  joined 
together  side  by  side  and  end  to  end  by  peculiar 
cementing  substances. 

In  addition  to  these  markings  on  the  fibres,  if 
high  magnifying  powers  are  used  and  the  fibre  is  in 
a  state  of  extension,  a  fine  line  is  seen  crossing  the 
fibre  through  the  centre  of  the  light  transverse 
band  ;  this  corresponds  with  the  dividing  line 
between  the  ends  of  the  sarcous  elements,  and  is 
called  Krauses  line.  Under  favorable  conditions 
the  dark  band  is  also  seen  to  be  crossed  by  a  line 
called  Henseiis  line,  whose  nature  is  as  yet  but 
imperfectly  understood. 

All  of  the  above-described  structural  features  of 
the  muscular  fibres  are  much  more  distinct  after 
treatment    with    chemical    agents,    and     after    the 


98  NORMAL  HISTOLOGY. 

death  of  the  tissue ;  the  longitudinal  striation  is 
not  visible  during  life,  and  the  distinct  separation 
of  the  primitive  fibrillae  and  discs  can  only  be 
accomplished  by  chemical  means.  We  can  see  the 
various  markings  with  sufficient  clearness,  on  the 
fresh  or  living  muscle,  to  convince  ourselves  that 
marked  optical  differences,  at  least,  exist  in  different 
parts  of  the  fibres ;  but  that  the  living  fibre  is  made 
up  of  distinct  elements,  having  the  structure  which 
we  see  in  the  isolated  or  partially  isolated  sarcous 
elements,  although  probable,  is  by  no  means  proven, 
since  this  isolation  by  chemical  means  may  signify 
only  a  tendency  to  break  up  in  certain  directions, 
and  not  a  definite,  preexisting  separate  structure. 

2.  The  nuclei,  which  in  the  mammalia  lie  upon 
the  surface  of  the  fibres,  and  directly  beneath  the 
sarcolemma,  in  the  amphibia,  fishes,  and  certain 
birds,  also  embedded  within  the  contractile  substance, 
are  usually  large,  flat,  and  ellipsoidal  in  shape,  con- 
tain nucleoli,  and  lie  with  their  long  axes  coinci- 
dent in  direction  with  the  axis  of  the  fibres.  They 
are  irregularly  scattered  along  the  fibre,  and  a  small 
amount  of  granular  matter  is  usually  seen  in  their 
immediate  vicinity. 

3.  The  sarcolemma,  a  delicate,  structureless, 
membranous  sheath,  is  so  thin,  and  so  closely  en- 
closes the  contractile  substance  and  nuclei,  that  we 
cannot  usually  see  it,  unless  we  separate  it  by 
artificial   means    from     the    underlying    structures. 


MUSCULAR   TISSUE.  99 

Where  the  muscular  fibres  join  tendons,  the  sarco- 
lemma  ends  in  the  form  of  a  pointed  or  rounded 
blind  sac,  to  which  the  tendon-fibres  are  attached. 

The  muscular  fibres  lie  closely  packed  together, 
their  ends  lapping  over  on  to  adjacent  fibres,  and 
forming  bundles  which  are  enclosed  in  sheaths  of 
connective  tissue.  Such  bundles  are  again  grouped 
to  form  larger  bundles,  and  thus  the  larger  and 
smaller  muscular  bellies  and  bands  are  formed. 
Arteries  enter  the  muscular  bundles  and  break  up 
into  capillaries,  which  run  along  the  fibres,  forming 
a  long  and  narrow-meshed  net.  Motor  nerves  also 
pass  into  the  muscles,  divide  and  subdivide,  and 
terminate,  at  the  surface  of  the  individual  fibres,  in 
structures  called  motor  end  plates. 

PRACTICAL   STUDY. 

Fresh  Muscle. — A  very  small  bit  is  dissected  off,  with 
as  little  stretching  as  possible,  from  one  of  the  voluntary- 
muscles  of  a  recently-killed  mammal,  and  carefully  teased 
longitudinally  in  salt  solution.  In  such  a  preparation  the 
transverse  bands  and  indistinct  longitudinal  striations  will 
be  seen,  and  here  and  there,  where  the  needles  have 
pressed  on  the  fibres,  the  contractile  substance  may  be 
seen  to  have  been  broken  across  and  the  broken  ends  to 
have  retracted  within  the  sarcolemma,  leaving  the  latter 
as  a  clear  and  sometimes  folded  membrane  stretching 
across  the  interval.  At  the  cut  ends  of  the  fibres  the 
contractile  substance  will  often  be  seen  swelling  and  ex- 
truding from  the  sarcolemma  in  the  form  of  an  obscurely 


100  NORMAL  HISTOLOGY. 

striated  fungiform  mass.  Here  and  there  nuclei  are  seen  ; 
but  they  maybe  brought  much  more  clearly  into  view  by 
allowing  a  drop  of  two-per-cent.  acetic  acid  to  flow  under 
the  cover-glass  ;  then  the  contractile  substance  swells  and 
becomes  transparent,  the  striations  becoming  indistinct 
as  the  somewhat  shrunken  nuclei  become  more  clearly 
denned. 

Muscular    Fibres   Hardened  with    Osmic    Acid. —  The 
above-described  finer    structural    details    of    the    muscle 
fibres  are  much  more  evident  when  they  are  in  a  state  of 
extension  than  when   contracted.     We  may   render  this 
condition  permanent  for  study,  by  the  following  method  : 
the  skin  is  quickly  removed  from  the  leg  of  a  freshly-killed 
animal  (rabbit  or  dog),  and  one  of  the  large  muscles  of 
the  tkigh  is  forcibly  extended  with  the  fingers  ;  the  canula 
of  a   hypodermic   syringe  is  then  thrust  into  the  muscle, 
and  an  interstitial  injection  is  made  of  a  mixture  of  equal 
parts  of   i-per-cent.    solution   of  osmic   acid   and  strong 
alcohol.     This  fluid,  in   three  or  four  minutes,  fixes   the 
fibres  in  the  extended  condition.    A  small  bit  of  that  por- 
tion  which  has  become  brown  is   now  snipped  off   and 
carefully  teased  and  mounted  in  a  mixture  of  equal  parts 
of  glycerin  and  water.     In  such  a  preparation  some  of  the 
fibres    frequently    escape    extension,    in    which  case,  the 
marked  difference  maybe  observed  between  the  extended 
and  non-extended  condition  of  the  fibres. 

Sections  of  Hardened  Muscle. — The  details  of  the  struc- 
ture of  muscular  fibres,  as  well  as  their  grouping  and 
relation  to  the  connective  tissue,  may  be  well  studied  in 
sections  from  hardened  muscle.  For  this  purpose  the 
tongue  of  some  animal — such   as  the  dog — is  well  suited, 


rSCULAR   TISSUE.  IOI 

since  here  we  have  short  muscular  fibres  running  in  vari- 
ous directions  and  attached  to  tendons,  and  we  see  in  a 
_:le  transverse  section  of  the  organ,  at  once,  longitudi- 
nal and  transverse  sections  of  the. fibres.  The  tongue  of 
a  dog  is  hardened  in  bichromate  of  potassium  and 
alcohol,  and  tr  e  sections  through  the  anterior  half 

of  the  organ  are  stained  double  and  mounted  in  glycerin. 

In  muscular  tissue  thus  hardened,  the  primitive  fibrillar 
are  loosened  from  one  another,  and  in  some  parts  of  the 
specimen  are  usually  more  or  less  separated.  The  con- 
tractile substance,  morever,  usually  shrinks  away  some- 
what from  the  sarcolemma,  which  then  appears  in  the 
transverse  section  as  a  delicate  ring  around  the  fibre. 

Blood-vessels  are  seen  in  the  above  preparation,  but 
they  may  be  much  better  demonstrated  in  longitudinal 
sections  of  a  muscle  whose  vessels  have  been  injected. 

I.    b. INVOLUNTARY    STRIATED    OR    HEART- MUSCLE. 

In  mammalia,  the  heart-muscle  differs  in  several 
important  structural  features  from  the  \oluntary 
muscle. 

The  contractile  substance  has  essentially  the 
same  structure  as  the  latter,  but,  instead  of  being 
arranged  in  the  form  of  elongated,  unbranched  cylin- 
ders or  fibres,  without  distinct  cell-structure,  in  the 
heart-muscle  the  fibres  send  off  at  frequent  intervals 
short,  narrow  processes,  which  join  neighboring 
fibres,  forming  a  narrow-  and  long-meshed  net. 
Further,  the  fibres  which,  owing  to  the  numerous 
anastomoses,  are  very  irregular  in  form,  are  made  up 


102  NORMAL  HISTOLOGY. 

of  distinct  segments  or  cells,  each  segment  being 
cemented  at  the  ends  to  its  neighbors,  and  furnished 
with  a  flat,  elongated,  ovoidal,  or  often  rectangular 
nucleus.  In  the  vicinity  of  the  nuclei  we  usually  see 
a  certain  amount  of  granular  material  or  pigment 
particles.  The  nuclei  instead  of  lying,  as  in  the 
voluntary  muscles,  at  the  surface  of  the  contractile 
substance,  are  embedded  within  it.  We  are  unable, 
in  heart-muscle  to  demonstrate  a  sarcolemma.  The 
fibres  are  grouped  in  bundles,  which  are  enclosed  in 
connective  tissue  and  supplied  with  blood-vessels 
and  nerves. 

PRACTICAL   STUDY. 

Sections  of  Heart-muscle. — A  bit  of  the  heart  of  man,  or 
any  mammal,  should  be  hardened  in  Miiller's  fluid  and 
alcohol.  Longitudinal  and  transverse  sections  are  stained 
double  and  mounted  in  glycerin. 

Teased  Heart-muscle. — To  demonstrate  the  segments 
or  cells  of  which  the  heart-fibres  are  composed,  a  mod- 
erately thin  bit  of  fresh  heart-muscle  is  put  for  twenty- 
four  hours  into  a  dilute  solution  of  chromic  acid  (i  to 
5,000),  then  carefully  washed  with  pure  water  and  stained 
witli  picro-carmine.  It  is  then  teased  on  a  slide  and 
mounted  in  glycerin,  to  which  a  little  formic  acid  has 
been  added  (1  to  100).  The  formic  acid  renders  the  tis- 
sue more  transparent,  and  brings  more  clearly  into  view 
the  narrow,  jagged  lines  of  strongly  refractive  substance 
which  indicate  the  longitudinal  boundaries  of  the  muscle- 
cells. 


CHAPTER  VII. 

NERVE-TISSUE. 

The  primary  structural  element  in  nerve-tissue  is 
the  nerve-cell.  Nerve-cells  have  the  most  diverse 
forms,  and  always  possess  one  or  more  branching  or 
unbranched  processes.  In  certain  cells  the  un- 
branched  processes  are  extremely  long,  become  as- 
sociated with  other  tissue  elements,  and  constitute 
the  nerve-fibres.  Both  nerve-cells  and  their  processes, 
nerve-fibres,  are  enclosed  and  supported  by  pecu- 
liarly arranged  connective  tissue,  and  supplied  with 
blood  and  lymphatic  vessels.  The  nerve-fibres  form, 
for  the  most  part,  the  white  matter  of  the  nerve- 
centres  and  the  peripheral  nerves,  while  the  cells 
enter  largely  into  the  composition  of  the  gray  mat- 
ter. In  studying  nerve-tissues,  we  have  then  to 
consider : 

I.  Nerve-fibres,   and    the    supporting   connective- 
tissue  structures,  with  their  accessories. 

II.  Nerve-cells. 

I. NERVE-FIBRES,    ETC. 

These   are   of  two  kinds:  A,   mcdullated,   and   B, 
non-medullated.      This  distinction  corresponds  with 

103 


104  NORMAL  HISTOLOGY. 

the  physiological  and  anatomical  classification  of 
nerve-tissues  into  those  of  the  cerebro-spinal  and 
the  sympathetic  systems  ;  the  medullated  belonging 
to  the  former,  the  non-medullated  to  the  latter. 

A.  Medullated  Nerve-fibres. 

If  we  disregard  for  the  moment  the  structure  of 
these  nerves  at  their  point  of  origin  in  the  nerve- 
centres,  and  at  their  termination  in  the  periphery, 
and  study  their  structure  as  it  is  seen  in  the  con- 
tinuity of  any  of  the  larger  or  smaller  nerves,  we 
find  that  the  individual  fibres  present  three  distinct 
structural  elements:  I,  the  axis  cylinder;  2,  the 
medullary  sheath  ;  3,  the  7ieurileinrna. 

1.  Running  through  the  axis  of  the  fibre  is  a 
cylindrical,  with  high  powers,  delicately  longitudi- 
nally striated  structure — the  axis  cylinder.  This 
is  believed  to  be  the  essential  nerve-element  of 
the  fibre — the  process  of  the  nerve-cell,  from  which 
it  passes  without  break  of  continuity  to  the  periph- 
ery ;  and  it  is  probable  that  the  longitudinal  stri- 
ations  are  the  expression  of  its  composition  from 
still  finer  primitive  fibrils,  which,  as  we  shall  see 
when  we  study  the  nerve-cells,  seem  to  be  continued 
on  into  the  cell-body  itself,  within  the  nerve-cen- 
tres. 

2.  Closely  surrounding  the  axis  cylinder  is  a  tube 
or  sheath  of  varying  thickness — the  medullary 
sheath — composed  of  a  white,  semi-fluid,  translucent, 
strongly  refractive  substance,   called   myelin,  which 


NER I  rE-  7  VSS  UE.  I O  5 

undergoes  rapid  changes  after  death,  or  on  removal 
from  the  animal,  and  swells  and  assumes  a  multitude 
of  bizarre  forms  on  addition  of  water;  it  is  soluble 
in  alcohol,  chloroform,  and  ether;  and,  like  fat,  is 
hardened  and  turned  black  under  the  action  of  osmic 
acid.  The  medullary  sheath  does  not  form  a  con- 
tinuous tube,  but  at  tolerably  regular  intervals  is 
separated  into  segments. 

3.  The  neurilemma  or  sheath  of  Schwann  is  an 
extremely  thin,  structureless,  membranous  tube, 
which  tightly  encloses  the  medullary  sheath,  and, 
like  the  latter,  is  broken  up  into  segments.  At  the 
ends  of  the  segments  is  a  constriction  around  the 
fibre,  at  the  expense  of  the  medullary  sheath,  and 
the  ends  of  the  neurilemma  segments  are  joined 
together  by  a  thin  layer  of  cement  substance,  which 
extends  inward  to  the  axis  cylinder.  There  is  rea- 
son to  believe  that  the  neurilemma  extends  inward 
and  between  the  medullary  sheath  and  the  axis 
cylinder,  entirely  enclosing  the  segments  of  the  me- 
dullary sheath. 

Within  each  neurilemma  segment,  called  inter  annu- 
lar segment \  and  about  midway  between  the  constric- 
tions, lies  a  flattened,  elongated,  elliptical  nncleus. 
We  may  regard  the  neurilemma  segments,  with 
their  nuclei,  as  cylindrical  cells  cemented  together, 
end  to  end,  enclosing  the  segments  of  the  medullary 
sheath  and  surrounding  the  axis  cylinder,  the  latter 
passing  uninterruptedly  through  the  axis  of  the  seg- 


I06  NORMAL  HISTOLOGY. 

ments.  In  addition  to  these  structural  features,  we 
find,  on  examining  with  high  powers,  irregularly 
scattered  along  each  interannular  segment,  delicate- 
oblique  lines  or  fissures,  called  the  incisures  of 
Schmidt,  which  seem  to  pass  from  the  neurilemma 
at  the  surface  inward  to  the  axis  cylinder,  obliquely 
through  the  medullary  sheath.  Their  significance 
is  not,  as  yet,  definitely  determined.  Medullated 
nerve-fibres  vary  greatly  in  diameter. 

Connective  Tissue  of  the  Nerves. — The  nerve-fibres 
are  bound  together  by  connective  tissue,  to  form 
larger  and  smaller  nerve-fascicles,  which,  singly  or  in 
bundles,  we  usually  call  simply  nerves.  If  we  follow 
the  nerves  outward  toward  their  peripheral  termina- 
tions, we  find  that  they  divide  and  subdivide,  be- 
coming, as  they  do  so,  smaller  and  smaller,  until  we 
finally  come  to  nerves  which  consist  of  a  single 
fibre.  These  single  nerve-fibres  do  not  lie  free  in 
the  tissues,  but  are  enclosed  in  a  distinct  sheath, 
called  Henle ' s  slieatli,  which  is  a  tube  formed  of  a 
single  layer  of  endothelial  cells,  placed  edge  to  edge, 
and  cemented  together.  Between  the  sheath  and 
the  nerve-fibre  is  a  narrow  space,  which,  under  nor- 
mal conditions,  is  filled  with  lymph. 

Having  become  acquainted  with  this  simple  struc- 
ture of  the  single  terminal  nerves,  let  us  follow  them 
backward.  We  find,  as  we  do  so,  that  as  they  be- 
come larger  by  the  junction  of  several  fibres  a  small 
amount   of    fibrillar    connective    tissue   appears   be- 


N E  A'  /  rE-  77 SS  C/E.  I O  7 

twcen  the  fibres  within  Henle's  sheath,  and  that  the 
latter  becomes  attached  to  the  surrounding  struc- 
tures by  a  layer  of  connective  tissue.  Finally,  when 
we  arrive  at  the  larger  nerve-trunks,  we  find  that  in 
each  the  connective  tissue  presents  itself  in  three 
ways  : 

1.  It  forms  a  distinct  sheath,  which,  although  the 
analogue  of  Henle's  sheath,  has  a  much  more  com- 
plicated structure,  and  is  called  the  lamellar  sheath. 
This  is  composed  of  several  concentric  lamellae,  each 
of  which  is  formed  of  a  fenestrated  membrane  of 
fibrillar  connective  tissue  containing  granules  of 
elastic-tissue  substance,  and  covered  with  endothe- 
lial cells.  The  lamellae  are  connected  by  oblique 
fibres,  which  pass  from  one  to  the  other,  binding 
them  more  or  less  firmly  together.  The  whole  forms 
a  compact  sheath,  closely  investing  the  fascicle  of 
nerve-fibres. 

2.  Outside  of  the  lamellar  sheath,  and  joining  it 
to  adjacent  structures — to  neighboring  nerve-fasci- 
cles, if  the  nerve-trunk  is  composed  of  several  of 
these,  as  is  the  case  in  many  large  nerves — we  find 
loose  fibrillar  connective  tissue  with  flattened,  ir- 
regular-shaped cells — like  those  found  in  the  loose 
subcutaneous  connective  tissue — reinforced  by  elas- 
tic fibres,  and  often  containing  fat-cells.  The  fibril 
lated  and  elastic  fibres,  especially  in  the  immediate 
vicinity  of  the  lamellar  sheath,  usually  run  in  a  di- 
rection approximately  parallel  with  the  axis  of    the 


I08  NORMAL  HISTOLOGY. 

nerve.    This  tissue  is  called  the  peri-fascicular  connec- 
tive tissue. 

3.  We  find  within  the  lamellar  sheath  and  be- 
tween the  nerve-fibres  composing  the  fascicle,  in  the 
first  place,  prolongations  inward  of  the  tissue  com- 
posing the  lamellar  sheath  ;  and,  second,  fine  fibril- 
lated  fibres  and  flattened  cells  which  lie  in  the 
interstices  between  the  nerves  and  fibres.  This  tis- 
sue is  called  the  intra-fascicular  connective  tissue. 
Blood-vessels  penetrate  the  lamellar  sheath  of  the 
medium-sized  and  larger  nerves,  and  a  very  long- 
meshed  and  abundant  capillary  net-work  is  formed 
in  the  intrafascicular  connective  tissue.  Lymphatic 
channels  and  spaces  are  also  abundant  within  the 
nerves,  so  that  the  fibres  are  bathed  in  nutritive 
fluids. 

Termination  of  Medullated  Nerve- fibres 

1.  /;/  the  Nerve-centres. — We  find  in  the  nerve- 
centres,  nerves  which  have  no  neurilemma,  and 
others  in  which  both  neurilemma  and  medullary 
sheath  fail — the  so-called  naked  axis-cylinders ;  we 
find,  further,  extremely  delicate  filiform  'structures 
which  are  believed  to  be  the  primitive  nerve-fibrils. 
The  axis-cylinders  of  the  nerves  being,  as  above 
stated,  processes  of  nerve-cells,  the  nerve-fibres,  as 
we  trace  them  back  into  the  centres,  must  sooner  or 
later  join  cells.  Their  exact  mode  of  connection 
with  the  cells  is  not  in  all  cases  sufficiently  well  un- 


RVE-TISSUE.  109 

derstood  ;  but  it  is  believed  that  they  either  join 
the  cells  in  the  form   of  naked  axis-cylinders,  or,  in 

other  cases,  that  the  axis-cylinders  break  up  into 
their  constituent  primitive  fibrils  before  entering  the 
cells. 

2.  In  the  Periphery. — The  peripheral  termination 
of  nerves  is  a  subject  which  presents  extreme  diffi- 
culties to  the  histologist,  and  with  few  exceptions 
the  exact  way  in  which  this  occurs  is  unknown.  The 
motor  nerves,  which  are  distributed  in  the  voluntary 
striated  muscles,  terminate  in  distinct,  nucleated, 
finely  granular  structures  on  the  surface  of  the 
fibres,  called  end  plates ;  those  which  go  to  the 
smooth  muscle  tissue  break  up  into  fine  plexuses, 
from  which  fibrils  seem  to  pass  either  to  the  individ- 
ual muscle-cells  or  to  the  surface  of  cell-bundles.  In 
the  case  of  some  of  the  nerves  of  special  sense,  we 
have  elaborate  nerve-structupes  such  as  the  retina, 
auditory  apparatus,  etc.  Again,  we  find  the  nerves 
ending  in  small,  complex,  isolated  bodies,  such  as  the 
.  so-called  tactile  corpuscles,  etc.  In  some  cases  as 
the  nerves  approach  their  peripheral  terminations, 
they  lose  the  medullary  sheath  and  neurilemma,  and 
the  axis-cylinder  breaks  up  into  very  numerous, 
exceedingly  delicate  fibrils  which  sometimes  form  in- 
tricate plexuses ;  some  of  the  fibrils  appear  to 
terminate  by  free  extremities ;  others,  it  is  probable, 
end  in  single  cells  of  various  kinds  ;  but  the  whole 
subject  of  peripheral  nerve-endings  is  far  too  intru 


I  I O  NORMA  L  HIS  TO  LOG  V. 

cate  and  too  little  understood,  to  demand  more  than 
a  passing  mention  in  a  course  of  study  as  elemen- 
tary as  that  which  now  engages  us. 

b .  — Non-u  i  c  dull  a  ted  Nerve -fib  res. 

These  are  also  called  fibres  of  Rcviak.  Unlike 
the  nerve-fibres  which  we  have  just  been  studying, 
they  possess  no  medullary  sheath  and  no  neuri- 
lemma. They  are  simply  grayish  translucent  cords 
of  varying  diameter  ;  they  are  indistinctly  longitu- 
dinally striated,  and  are  intimately  connected  with 
one  another  by  frequent  inosculations.  The  fibres 
seem  to  divide  and  send  off  oblique  branches  to  join 
neighboring  fibres.  Flattened,  elongated  nuclei  lie 
at  frequent  intervals  upon  the  surface  of  the  fibres. 
These  fibres  considerably  resemble,  in  their  general 
appearance,  the  fibrillated  fibres  of  ordinary  connec- 
tive tissue,  but  careful  examination  shows  them  to 
be  entirely  distinct  structures. 

They  are  grouped  in  bundles  to  form  nerves, 
sometimes  alone,  but  very  frequently  in  connection 
with  medullated  nerve-fibres.  Thus,  in  the  pneumo- 
gastric,  we  find  a  considerable  part  of  the  fibres  to 
be  non-medullated,  and  intimately  bound  in,  by  the 
intrafascicular  connective  tissue,  with  medullated 
fibres.  The  non-medullated  fibres  originate  in 
nerve-cells  of  a  peculiar  structure,  to  be  presently 
described  ;  but  of  their  peripheral  terminations  we 
know  almost  nothing. 


NER  VE-  TISS  UE.  1 1 1 


II.    NERVE-CELLS. 


Nerve-cells,  ox  ganglion-cells,  as  they  are  frequently- 
called,  although  presenting  the  greatest  diversity  in 
form,  have  yet  some  quite  distinctive  characters  in 
common.  The  cell-body  is  finely  granular  and 
delicately  striated,  often  containing  pigment-gran- 
ules. The  nucleus  is  large,  well-defined,  vesicular  in 
appearance,  and  usually  contains  a  large  shining 
nucleolus.  They  all  have  at  least  one  process,  most 
of  them  have  more  ;  and  they  are  hence  often 
classified  as  unipolar,  bipolar,  multipolar  ganglion- 
cells.  The  above-mentioned  striations  in  the  cell- 
body  are  often  seen  to  continue  out  into  the 
processes,  and  are  apparently  continuous  with  the 
striations  or  fibrils  of  the  axis-cylinder  of  the 
nerves. 

In  many  nerve-cells,  especially  in  the  spinal  cord, 
we  recognize  two  distinct  kinds  of  processes :  first, 
those  which,  soon  after  leaving  the  cell,  divide  and 
subdivide  until  they  become  extremely  fine  and 
delicate,  and,  in  some  cases,  seem  to  join  equally 
fine  processes  of  other  cells — such  delicate  cell- 
processes  make  up  a  considerable  portion  of  the 
gray  matter  of  the  cord,  and  are  called  branching 
processes  ;  second,  such  as  pass  off  from  the  cell,  and, 
without  dividing,  presently  are  surrounded  by  a 
sheath  of  myelin,  and  become  medullated  nerve- 
fibres  ;  the  latter  are  called  axis-cylinder  processes. 


1 1 2  NORMA  L  HIS  TO  LOG  Y. 

Nerve-cells  vary  greatly  in  size,  and  although  the 
forms  which  they  present  are  most  diverse,  we  yet 
find  that  a  considerable  proportion  of  those  found 
in  different  parts  of  the  nerve-centres  have  certain 
broadly  typical  forms.  Thus,  among  the  cells  in 
the  gray  matter  of  the  spinal  cord,  we  find  larger 
and  smaller  fusiform  or  spheroidal  branching  cells, 
and,  which  are  more  characteristic,  large,  irregular- 
shaped  cells,  with  several  branching  processes  and  a 
well-defined  axis-cylinder  process.  In  the  cortex  of 
the  cerebrum,  while  we  find  variously  shaped 
larger  and  smaller  cells,  we  find  also  characteristic 
pyramidal  cells  of  varying  size,  which  give  off 
processes  from  both  the  base  and  apex.  In  the 
cerebellum,  we  find  just  at  the  inner  edge  of  the 
gray  cortical  matter,  irregular  globular  or  ovoidal 
cells,  which,  from  the  side  toward  the  surface  of  the 
brain,  send  off  one  or  two  branching  processes ;  on 
the  opposite  side  we  can  usually  demonstrate  the 
commencement  of  a  single  delicate  process,  which 
is  supposed  to  correspond  to  the  axis-cylinder 
process,  though,  since  it  almost  invariably  breaks  off 
near  the  cell  in  the  attempt  to  isolate  the  latter,  its 
nature  is  not  yet  definitely  determined.  These 
celis  are  called  Purkinje  s  cells. 

The  ganglion-cells  of  the  sympcitlictic  are  usually 
globular  or  ovoidal,  and  are  peculiar  in  that  each 
cell  is  surrounded  by  a  distinct  capsule  of  connective 
tissue,  lined   with   flattened  cells,  resembling  endo- 


NERVE-TISSUE.  113 

thelium.  They  have  one  or  more  processes  which 
pierce  the  capsule  and  become  non-medullatcd 
nerve-fibres. 

PRACTICAL    STUDY. 

Fresh  Nerve. — A  bit  of  fresh  nerve — the  sciatic  of  the 
frog  answers  very  well — should  be  carefully  and  rapidly 
teased  apart  longitudinally,  in  one-half-per-cent.  salt  so- 
lution— care  being  taken  to  pull  apart  the  fibres  from  the 
ends,  so  as  to  break  them  as  little  as  possible, — and  cov- 
ered ;  pressure  from  the  cover-glass  being  avoided  by 
placing  a  bit  of  paper  or  hair  beside  the  specimen.  The 
nerve-fibres  present,  if  examined  at  once,  in  many  parts,  a 
sharp  and  regular  double  contour,  which  is  their  normal 
appearance,  and  along  their  course  the  constrictions  and 
nuclei  may  here  and  there  be  seen.  The  axis-cylinder 
and  neurilemma  are  for  the  most  part  invisible  ;  the  for- 
mer owing  to  the  lack  of  transparency  in  the  medullary 
sheath,  the  latter  because  of  its  extreme  thinness  and 
close  contact  with  the  medullary  sheath.  Very  soon,  at 
once  in  some  parts  of  the  specimen,  the  contours  of  the 
fibres  will  be  seen  to  become  irregular,  the  myelin  shrink- 
ing away  at  some  parts  from  the  neurilemma,  and 
swelling  out  at  others.  At  the  severed  ends  of  the  fibres 
the  myelin  will  be  seen  welling  out  from  the  neuri- 
lemma, and  breaking  off  into  the  fluid  in  irregular  glob- 
ular or  contorted  masses. 

After  the  swelling  and  irregular  breaking  up  of  the 
myelin  has  occurred — this  may  be  hastened  by  allowing 
water  to  run  under  the  cover-glass — the  neurilemma  may 
be  seen  here  and  there  stretching  across  between  the  va- 


1 1 4  NORMA  L  HIS  TO  LOGY. 

ricosities  formed  by  the  swollen  myelin,  and  either  at  the 
broken  ends  of  the  fibres  or  along  their  course,  the  axis- 
cylinder  may  occasionally  be  seen. 

Nerve-fibres  Treated  with  Osmic  Acid. — The  most 
complete  demonstration  of  the  nerve-fibre  may  be  ob- 
tained by  treatment  with  osmic  acid.  This  agent  fixes 
the  myelin  and  other  constituents  of  the  fibre  nearly  in 
their  normal  form,  staining  the  myelin  black.  In  apply- 
ing this  agent  it  is  necessary  to  maintain  the  nerve  in  a 
state  of  gentle  tension,  because  it  is  otherwise  somewhat 
contracted  and  distorted.  This  is  done  by  gently  stretch- 
ing the  nerve — the  sciatic  of  the  rabbit  will  answer — along 
a  bit  of  wood  which  has  been  whittled  away  at  one  side, 
so  that  the  nerve  may  lie  free.  It  is  fastened  by  the  ends 
to  the  wood  by  threads.  The  nerve  thus  prepared  is  im- 
mersed for  twenty-four  hours  in  an  aqueous  solution  of 
osmic  acid  (i  to  ioo),  then  washed,  and  a  small  bit  care- 
fully teased  apart  longitudinally  in  glycerin.  In  such  a 
preparation  nearly  all  the  structures  in  the  fibre  can  be 
readily  seen  :  the  constrictions  and  nuclei,  the  medullary 
sheath  stained  black,  the  incisures  of  Schmidt,  and  where, 
as  will  almost  always  occur  in  some  parts  of  the  specimen, 
the  medullary  sheath  has  been  broken  across,  or  the  seg- 
ments pulled  asunder,  or  the  myelin  has  contracted  at 
the  constrictions,  the  neurilemma  and  axis-cylinder.  Not 
infrequently,  if  the  teasing  has  not  been  very  carefully  done, 
the  segments  of  the  medullary  sheath  are  broken  across 
in  many  places  and  separated,  giving  the  fibre  a  beaded 
appearance. 

Transverse  Sections  of  JVerves  Stained  with  Osmic  Acid. 
— A  nerve  treated  as  above  with   osmic  acid  is  not  firm 


NER  VE-  TISS  UE.  1  I  5 

enough  to  permit  the  making  of  thin  sections,  and  should 
be  hardened  by  what  is  known  as  the  gum  embedding 
method,  as  rollers  :  a  bit  of  the  nerve,  after  the  osmic 
acid  treatment,  is  immersed  for  twenty-four  hours  in  al- 
cohol, soaked  for  a  few  moments  in  water,  to  remove  the 
alcohol,  and  immersed  for  twenty-four  hours  in  a  syrupy 
solution  of  gum-arabic,  then  put  again  for  twenty-four 
hours  in  alcohol.  By  this  process  the  gum  is  precipitated 
by  the  alcohol  in  solid  form  in  the  interstices  of  the  fibres, 
giving  it  sufficient  consistence  for  cutting.  The  sections 
(which  must  be  very  thin),  either  stained  with  picro-car- 
mine  or  unstained,  are  mounted  in  glycerin.  In  such  a 
preparation  the  uncolored  axis-cylinder  is  seen  sur- 
rounded by  a  black  ring,  the  medullary  sheath,  and  here 
and  there  the  nuclei  of  the  neurilemma  are  seen  at  the 
edge  of  the  fibres.  The  connective  tissue  surrounding 
the  fibres  has  a  grayish  color.  The  nerve-fibres  will  be 
seen  to  have  varying  diameters  and  to  present  marked 
differences  in  form,  some  of  them  depending  upon  artifi- 
cial changes,  others  upon  the  difference  in  level  at  which 
the  fibres  have  been  cut  across. 

Transverse  Sections  of  Nerves  Preserved  in  Chromic 
Acid. — Bits  of  the  sciatic  nerve  from  the  rabbit  or  any 
other  mammal  should  be  lightly  stretched  along  a  bit  of 
wood  and  placed  in  a  solution  of  chromic  acid  (i  to  500); 
in  two  weeks  it  is  washed  and  transferred  to  alcohol.  It 
is  now  embedded  in  gum  (see  above),  and  thin  transverse 
sections  made  and  stained  double  and  mounted  in  bal- 
sam. In  such  preparations  the  general  relation  of  con- 
nective tissue  to  the  nerve-fibres  is  well  seen. 

Nerve-fibres    Treated  with  Nitrate  of   Silver. — By  this 


1 1 6  NORMA L  HISTOLOGY. 

method  we  obtain  hints  concerning  the  structure  of  nerves 
which  are  of  no  little  significance  from  a  physiological 
point  of  view.  A  fresh  nerve  is  slightly  teased  apart  on 
a  slide,  a  large  drop  of  a  one-half-per-cent.  solution  of 
nitrate  of  silver  added  and  allowed  to  remain  for  four 
minutes  ;  this  is  washed  off  with  one-half-per-cent.  salt 
solution,  the  specimen  transferred  to  a  drop  of  glycerin 
on  another  slide,  the  fibres  carefully  teased  apart,  and 
covered.  The  preparation  is  now  exposed  to  sunlight  or 
diffuse  daylight  until  it  becomes  brown.  If  now  ex- 
amined, at  tolerably  regular  intervals  along  the  fibre,  tiny 
brown  or  black  crosses,  called  Ranviers  crosses,  will  be 
seen  ;  the  transverse  arm  of  the  cross  being  the  stained 
cement  substance  between  the  neurilemma  segments  at 
the  constrictions  ;  the  longitudinal  arm,  which  coincides 
with  the  axis  of  the  fibre,  and  which  is  longer  or  shorter, 
depending  upon  the  length  of  time  to  which  the  fibre  was 
exposed  to  the  action  of  the  silver,  is  the  axis- cylinder. 
If  the  specimen  be  allowed  to  remain  longer  than  the 
above  time  in  contact  with  the  silver,  the  longitudinal  arm 
of  the  cross  will  be  longer. 

It  is  to  be  observed  that  the  axis-cylinder  is  first  stained 
at  that  part  which  passes  through  the  constrictions,  and 
not  along  the  segments.  Certain  other  soluble  substances 
which  stain  the  axis-cylinder  comport  themselves  in  the 
same  way.  We  infer  from  this  that  at  the  constrictions 
certain  substances  in  solution  can  pass  into  the  fibre  and 
come  in  contact  with  the  axis-cylinder,  or  the  nerve- 
element  proper  of  the  fibre.  This  inference  is  significant 
in  connection  with  the  nutrition  of  the  nerves,  since  we 
are    justified    in    assuming    that    nutritive    substances    in 


NERVE-TISSUE.  WJ 

solution  may  pass  also  to  the  axis-cylinder  in  the  same 
way. 

It  is  not  improbable  that  the  constrictions  serve  yet  an- 
other important  purpose.  The  myelin  of  the  medullary 
sheath  being  a  semi-fluid  substance — perhaps  serving 
either  to  isolate  the  axis-cylinder  or  protect  it  from  ex- 
ternal violence — it  would  inevitably  tend  to  gravitate  to 
the  lower  parts  of  the  nerves,  were  it  not  that  it  is  held  in 
position  by  being  enclosed,  so  to  say,  in  cylindrical  cases, 
/.  <.,  the  neurilemma-cells,  between  the  constrictions. 

The  non-medullated  nerve-fibres  may  be  demonstrated 
in  connection  with  the  sympathetic  ganglion-cells  ;  see 
below. 

I.  Nerve-cells  :  a.  Spinal  Cord. — Small  bits  of  the  gray 
matter  from  the  spinal  cord  of  man,  or  from  the  ox  or 
sheep,  should  be  put  for  ten  days  in  a  dilute  solution  of 
chromic  acid  (one  to  five  hundred),  and  then  carefully 
shaken  in  a  test-tube  with  water  colored  lightly  with  car- 
mine ;  when  the  bits  have  become  thoroughly  broken  up 
into  small  particles,  the  tube  is  allowed  to  stand  for  a  day 
or  two,  until  the  particles  which  have  settled  to  the  bot- 
tom are  sufficiently  stained.  The  supernatant  fluid  is 
then  decanted,  and  with  a  glass  tube  a  small  drop  of  the 
disintegrated  tissue  is  conveyed  to  a  drop  of  glycerin  on 
a  slide  and  covered,  pressure  on  the  cells  being  avoided 
in  the  usual  way.  If  the  first  preparation  does  not  con- 
tain the  required  cells,  others  should  be  made.  In  this 
way,  if  the  shaking  be  carefully  done,  the  ganglion-cells 
are  freed  to  a  considerable  degree  from  the  surrounding 
parts,  and  such  may  be  found  as  present  numerous  long 
branching  processes,  as  well  as  the  axis-cylinder  process. 


1 1 8  NORMA L  HIS TOL OGY. 

In  addition  to  the  cells,  such  specimens  present  frag- 
ments of  connective-tissue,  bits  of  naked  axis-cylinders, 
and  medullated  nerve-fibres,  myelin-droplets,  etc. 

b.  Brain. — In  the  way  above  described,  cells  should  be 
prepared  from  the  gray  cortical  portion  of  the  cerebrum 
and  cerebellum. 

c.  Sympathetic. — For  the  demonstration  of  these  cells, 
and  the  fibres  connected  with  them,  the  frog  answers 
very  well.  The  animal  having  been  killed  by  breaking 
up  the  medulla,  the  abdominal  cavity  is  opened,  and  the 
intestines  and  liver  carefully  removed  ;  the  aorta  will 
then  be  seen  lying  along  the  vertebral  column.  The 
sympathetic  ganglia  and  nerves  lie  along  the  walls  of  the 
aorta,  and  in  the  tissue  surrounding  the  origin  of  the 
spinal  nerves.  The  head  and  forelegs  should  now  be 
cut  off  close  behind  the  latter,  and  the  hind  legs  severed 
close  to  the  body  ;  the  trunk  is  then  laid  in  a  small  dish, 
and  covered  with  equal  parts  of  one-per-cent.  solution  of 
osmic  acid,  alcohol,  and  water.  The  dish  is  covered  and 
set  aside  for  twenty-four  hours,  when  the  aorta,  together 
with  the  tissue  surrounding  the  commencement  of  the 
spinal  nerves,  is  dissected  off  in  a  single  piece,  spread  on 
a  slide,  and  examined  with  a  low  power.  Groups  of  sym- 
pathetic nerve-cells  are  seen  here  and  there  in  the  speci- 
men, and  are  readily  distinguished  by  the  orange  color  of 
the  cells  :  one  or  two  of  them  are  to  be  isolated  and 
freed  as  much  as  possible  from  the  enclosing  tissue,  care- 
fully teased  apart  on  a  slide,  stained  lightly  with  hema- 
toxylin and  then  with  eosin,  and  mounted  in  glycerin. 
Successful  preparations  will  show  not  only  the  nerve- 
cells  and  non-medullated  nerve-fibres,  but  also  the  con- 
nection of  the  two  within  the  capsule. 


CHAPTER  VIII. 

BLOOD-VESSELS— LYMPHATIC  VESSELS. 
BLOOD-VESSELS. 

Blood-vessels  are  of  three  kinds :  arteries,  veins, 
and  capillaries.  Although  merging  without  sharp 
demarcation  into  one  another,  these  vessels,  in  their 
typical  forms,  present  distinct  differences  in  struc- 
ture. The  capillaries  being  the  simplest,  it  will  be 
convenient  to  commence  with  them. 

If  we  examine  a  capillary  vessel,  either  fresh  or 
after  it  has  been  in  preservative  fluids,  it  presents 
the  appearance  of  a  narrow  tube,  with  very  thin, 
homogeneous  walls,  in  which,  at  frequent  intervals, 
elongated  nuclei  are  embedded,  their  long  axes 
being  parallel  with  the  axis  of  the  tube.  If,  how- 
ever, we  inject  the  vessels  with  a  dilute  solution  of 
nitrate  of  silver,  and  expose  them  to  the  light,  we 
find  that  the  inside  of  the  tube  is  divided  by  narrow 
black  lines  into  elongated,  irregular-shaped  spaces ; 
and  if  we  then  stain  the  specimen  with  hasmatoxy- 
lin,  we  find  that  a  nucleus  lies  in  each  space.  The 
walls  of  the  capillaries  are,  then,  not  formed  by  a 
homogeneous  membrane,  but  made  up  of  cells  having 
the  character  of  endothelium.      The  capillaries  are 

119 


120  NORMAL  HISTOLOGY. 

endothelial  tubes.  This  layer  of  endothelial  cells, 
which  alone  forms  the  walls  of  the  capillaries,  is 
found  lining  all  the  other  blood-channels,  arteries 
and  veins,  as  well  as  the  heart.  In  the  blood-vessels 
it  is  called  the  intima. 

If,  now,  we  follow  the  capillaries  in  a  direction 
toward  the  arteries,  we  find  that  the  connective 
tissue  in  which  they  lie  is  arranged  in  the  form  of  a 
thin  layer  along  their  walls.  This  layer,  which  is 
also  present  in  all  arteries  and  veins,  is  called  the 
adventitia.  Almost  as  soon  as  we  find  the  adven- 
titia,  we  notice  another  layer  between  it  and  the 
intima,  formed  of  a  single  row  of  smooth  muscle- 
cells,  or  of  scattered  cells,  wound  transversely  or 
obliquely  around  the  vessel.  This  layer  is  called 
the  media  or  musculosa,  and  a  vessel  having  these 
three  simple  layers  in  its  walls  is  called  an  arteriole. 
In  these  three  layers,  intima,  media,  and  adventitia, 
we  have  the  types  of  all  the  layers  which  occur  in 
the  walls  of  the  largest  and  most  complicated  blood- 
vessels. The  individual  layers  become,  indeed, 
more  complex  in  structure ;  but,  with  the  exception 
of  elastic  elements,  no  new  tissues  appear. 

Turning  now  to  a  larger  artery — the  radial,  for 
example — and  examining  the  various  layers  in  its 
walls,  we  find  that  the  intima  is  no  longer  formed 
of  a  simple  endothelial  tube,  but  that  outside  of 
this  a  new  layer  has  appeared,  composed  of  ill- 
defined  fibrillated  and  of  elastic  fibres,  among  which 


BLOOD-  VESSELS.  1 2 1 

lie  large,  flattened  branching  cells.  This  layer  is 
called  the  intermediary  layer  of  the  intima,  and  is 
sharply  separated  from  the  media  by  a  fenestrated, 
elastic  membrane,  called  the  membrana  elastica  in- 
timcc,  which  in  contracted  vessels  is  usually  folded. 
The  media  presents  here  quite  a  thick  layer  of 
smooth  muscle-cells,  passing  transversely  around 
the  vessel,  and  among  these  we  find  a  few  elastic 
fibres,  which  are  connected  with  the  elastic  elements 
in  the  intima  and  adventitia.  The  adventitia  is 
thicker,  and  consists  chiefly  of  fibrillar  connective 
tissue  with  elastic  fibres. 

In  the  larger  arteries,  such  as  the  carotids,  aorta, 
etc.,  we  find  that  the  individual  layers  are  consid- 
erably less  sharply  defined.  The  three  layers  of  the 
intima  are  much  less  distinct ;  in  the  media  the 
elastic  tissue  is  very  abundant,  taking  the  place,  to 
a  considerable  extent,  of  the  muscular  elements  ;  it 
is  arranged  in  irregular  lamellae  and  fibres,  between 
which  lie  fibrillated  fibres,  and  connective-tissue  and 
smooth  muscle-cells,  the  latter  no  longer  all  lying 
uniformly  transversely  to  the  axis  of  the  vessel.  In 
the  adventitia  also  of  the  large  vessels  the  elastic 
elements  are  very  numerous,  being  most  abundant 
in  the  vicinity  of  the  media.  In  the  adventitia  of 
some  of  the  large  vessels,  smooth  muscle-cells  occur, 
arranged  usually  with  their  long  axes  parallel  with 
the  axis  of  the  vessel. 

The  walls  of '  tlie  veins,  like  those  of  the  arteries, 


122  NORMAL  HISTOLOGY. 

consist  of  three  layers,  and  these  layers  have  in 
general  an  analogous  structure;  but  they  are 
neither  as  distinct,  nor  are  their  structural  features 
as  constant,  as  those  of  the  arteries.  Moreover,  we 
find  that  veins  of  the  same  calibre  present,  in  dif- 
ferent parts  of  the  body,  marked  differences  in 
structure,  and,  unlike  the  arteries,  the  thickness  of 
their  walls  is  not  uniformly  proportional  to  the 
calibre  of  the  vessel.  In  general  we  may  express 
the  structural  difference  between  veins  and  arteries 
by  saying  that  in  the  walls  of  the  former  the  elastic 
and  muscular  elements  are  much  less,  while  the 
connective-tissue  elements  are  more  abundant,  than 
in  the  walls  of  the  latter.  Now,  since  the  muscular 
and  elastic  elements  are  the  chief  constituents  of 
the  media,  and  the  fibrillar  connective-tissue  ele- 
ments, of  the  adventitia,  we  find,  in  general,  that  in 
the  veins  the  media  is  less,  while  the  adventitia  is 
more  developed.  The  intermediary  layer  of  the 
intima  is  entirely  absent  in  small  veins  ;  in  many  of 
medium  size  it  appears,  and  is  again  absent  in  the 
largest  vessels.  In  some  veins,  such  as  those  of  the 
bone,  central  nervous  system,  retina,  etc.,  the  mus- 
cular elements  are  almost  or  entirely  absent.  In 
certain  other  veins,  on  the  contrary,  such  as  the  v. 
portarum,  v.  renalis,  the  adventitia  contains  a  great 
abundance  of  muscular  elements  arranged  parallel 
with  the  axis  of  the  vessel. 

The  walls  of  the  larger  arteries  and  veins  contain 
blood  and  lymph-vessels,  called  vasa  vasorum. 


BLOOD-VESSELS.  1 23 

The  valves  of  the  veins  consist  of  bundles  of  fibril- 
lar connective  tissue  arranged  to  form  a  membran- 
ous projection  from  the  walls  of  the  vessels — the 
bundles  being  arranged,  in  general,  in  a  direction 
parallel  with  the  free  edge  of  the  valve.  They  con- 
tain also  a  net-work  of  elastic  fibres,  which,  at  that 
surface  of  the  valve  which  is  exposed  to  the  blood- 
current,  form  a  dense  layer  like  the  intermediary 
layer  of  the  vein-wall  itself.  The  whole  free  surface 
is  covered  with  endothelium  like  that  lining  the 
general  surface  of  the  vessel. 

ENDOCARDIUM  AND  VALVES  OF  THE  HEART. 

The  endocardium,  which  differs  somewhat  in  thick- 
ness and  structure  in  different  parts  of  the  heart, 
consists,  in  general,  of  a  membranous  expansion  of 
fibrillar  connective  tissue,  with  elastic  fibres  and 
smooth  muscle-elements,  which  lines  the  cavities  of 
the  heart,  and  is  covered  on  its  free  surface  with  a 
layer  of  endothelial  cells. 

If  we  examine  sections  made  perpendicular  to 
the  surface  of  the  endocardium,  we  find  that  just 
beneath  the  endothelium  is  a  layer  of  fibrillar  con- 
nective tissue,  with  flattened  cells,  reinforced  by  a 
net-work  of  elastic  fibres,  which  become  coarser  and 
more  abundant  in  that  portion  of  the  layer  lying  far- 
thest from  the  heart-cavities.  In  this  layer  are  smooth 
muscle-cells  running  in  various  directions.  Outside 
of  this  layer,  and  joining  it  to  the  muscular  tissue 


124  >RMAL  HISTOLOGY. 

proper  of  the  heart,  is  a  layer  of  loose  fibrillar  con- 
nective tissue,  in  which  the  blood,  lymphatic  ves- 
sels, and  nerves  lie  embedded. 

The  valves  of  the  heart  consist  of  fibrillar  connec- 
tive tissue,  arranged  in  membranes  or  fascicles,  and 
associated  with  elastic  tissue,  the  latter  being  most 
abundant  at  the  free  surfaces  of  the  valves ;  and 
here  it  is  present  in  the  greatest  quantity  and 
density  on  the  surface  which  is  most  directly  ex- 
posed to  the  current  of  blood — that  is,  on  the  auric- 
ular surfaces  of  the  tricuspid  and  mitral,  and  on 
the  ventricular  surfaces  of  the  aortic  and  pulmonary 
valves.  The  elastic  elements  are  also  more  abun- 
dant in  the  valves  of  the  left  than  of  the  right  side 
of  the  heart.  The  firmness  and  capacity  for  re- 
sistance of  the  elastic  tissue  being  borne  in  mind, 
the  significance  of  its  distribution  in  the  valves  will 
be  readily  perceived  ;  where  they  are  the  most  ex- 
posed to  the  impact  and  pressure  of  the  blood, 
there  they  are  the  most  firm  and  dense. 

LYMPHATIC    VESSELS. 

The  larger  lymphatic  vessels  have  a  structure 
quite  similar  to  that  of  the  veins,  and  like  the  latter 
they  are  supplied  with  valves.  They  approach  the 
arterial  type,  however,  in  that  the  muscular  fibres 
are  quite  abundant  in  proportion  to  the  thickness 
of  the  walls,  although  the  walls  themselves  are  very 
thin  in  proportion  to  the  size   of  the   lumen.     Fol- 


LYMPHATIC  VESSELS.  i2$ 

lowing  the  larger  lymphatics  toward  the  periphery, 
we  find  that  they  pass  over  into  irregular-branching 
and  pouching  channels,  the  walls  of  which  consist 
of  a  single  layer  of  endothelial  cells,  whose  edges 
are  very  sinuous,  dovetailing  into  one  another  like 
the  pieces  of  a  child's  puzzle-map.  These  channels 
are  called  lymphatic  capillaries.  Between  these  and 
certain  spaces  or  lacunae  in  the  tissues — those,  for 
example,  in  which  the  connective-tissue  cells  lie  (see 
page  44) — there  seems  to  be  a  direct  communica- 
tion, by  means  of  which  fluids,  and  probably  formed 
elements,  such  as  blood-cells,  pass  over  from  the 
blood  into  the  lymphatic  vessels. 

PRACTICAL   STUDY. 

Capillaries. — The  general  appearance  of  the  capillaries, 
as  well  as  of  the  smaller  arteries  and  veins,  is  best  seen  in 
those  parts  in  which  the  vessels  are  surrounded  by  but 
little  tissue,  as  in  thin  membranes  such  as  the  mesentery 
or  pia  mater.  A  slice  about  an  inch  thick  should  be 
made  from  the  surface  of  the  cerebrum  and  laid  for 
twenty-four  hours  in  Miiller's  fluid.1  A  small  fragment 
from  that  part  of  the  pia  which  dips  into  the  sulci  should 
now  be  carefully  separated  from  the  brain  substance, 
stretched  on  a  bit  of  thin  cork,  and  fastened  with  pins.  It 
is  then  laid  for  twenty-fours  each,  in  dilute  and  strong 
alcohol,  and  finally  stained  in  picro-carmine  and  mounted 
in  glycerin  ;  or  stained  double  and  mounted  in  balsam. 

1  The  normal  relations  of  the  parts  are  much  better  preserved  by 
a  preliminary  hardening  in  this  way  before  the  pia  is  removed  from 
the  brain,  than  if  it  is  removed  while  quite  fresh. 


126  NORMAL  HISTOLOGY. 

In  such  a  preparation,  although  we  see  the  elongated 
nuclei  in  the  capillary  wall,  we  cannot,  as  a  rule,  make 
out  the  outlines  of  the  cells.  To  accomplish  this  we  have 
recourse  to  the  use  of  dilute  solutions  of  nitrate  of  silver. 
This  can  be  applied  by  immersing  some  thin  membrane, 
such  as  the  mesentery,  for  an  hour  in  a  solution  of  nitrate 
of  silver  (1-500),  brushing  off  the  endothelium  from  the 
surface,  and  exposing  the  specimen  in  water  to  the  light, 
until  it  becomes  brown.  Or,  what  is  better,  the  entire 
vascular  system  of  a  small  animal,  such  as  a  frog,  may  be 
first  rinsed  out  with  water  through  a  canula  introduced 
into  the  aorta  and  attached  to  a  syringe,  and  then  injected 
with  the  above  silver  solution.  Thin  membranes,  such  as 
the  mesentery  or  bladder,  are  removed  from  the  animal, 
exposed  to  the  light  for  a  sufficient  time,  and  then,  either 
stained  or  unstained,  mounted  in  glycerin. 

Arteries  and  Veins. — Very  small  vessels  can  be  studied 
entire,  since  by  careful  focussing  we  can  bring  one  por- 
tion after  another  into  view,  obtaining  thus  what  are 
called  optical  sections. 

The  arterioles  may  be  prepared  by  pulling  out  some  of 
the  arteries  which  enter  the  brain  substance  ;  at  the  ends 
of  these  the  arterioles  into  which  they  divide  are  seen. 
Some  of  the  finer  twigs  are  cut  off,  stained  double, 
mounted  in  balsam,  and  studied  entire. 

In  larger  vessels  this  simple  method  is  no  longer  prac- 
ticable, and  we  have  to  resort  to  actual  sections.  Small- 
or  medium-sized  arteries  and  veins  may  be  prepared  by 
stretching  them,  when  fresh,  along  a  bit  of  wood,  with 
pins,  and  allowing  them  to  dry.  The  dried  vessels  are 
placed  between   two   bits   of   soft  cork  or  elder-pith,  and 


LYMPHATIC  VESSELS.  \2J 

thin  sections  shaved  of,  in  any  desired  direction,  with  a 
hard-tempered  dry  razor.  The  sections  swell  out  on  be- 
ing dropped  into  water  ;  they  are  then  stained  double 
and  mounted  in  balsam. 

Larger  vessels,  such  as  the  aorta,  vena  cava,  etc.,  may 
be  hardened  in  alcohol,  embedded  in  liver,  and  the  sec- 
tions stained  and  mounted  as  before. 

Heart-valves  and  Endocardium. — These  may  be  studied 
in  thin  sections  made  perpendicular  to  the  surface  of  the 
valves,  and  the  inner  surface  of  pieces  of  the  heart,  which 
have  been  stretched  on  cork  and  hardened  in  alcohol. 

Lymphatic  Vessels. — A  general  view  of  the  relation  be- 
tween the  larger  lymphatic  trunks  and  the  lymphatic 
capillaries,  is  best  obtained  by  studying  nitrate  of  silver 
prepartions  of  some  part  in  which  the  latter  are  abundant. 
Such  a  structure  we  find  in  the  tendinous  portion  of  the 
diaphragm — so  called  ce?itral tendon — of  the  rabbit,  which 
is  very  thin,  covered  on  both  sides  with  endothelium,  and 
very  richly  supplied  with  lymphatic  vessels. 

To  prepare  the  central  tendon,  a  rabbit  should  he 
killed  by  breaking  up  the  medulla,  and  the  thoracic  and 
abdominal  cavities  freely  opened  by  incisions  above  and 
below  the  anterior  line  of  attachment  of  the  diaphragm. 
Two  ligatures  are  passed  around  the  large  vessels  above 
the  diaphragm,  to  prevent  the  escape  of  blood  on  to  the 
part,  the  vessels  cut  across  between  them,  and  the  heart 
and  lungs  removed  from  the  chest.  The  suspensory  lig- 
ament of  the  liver  is  now  severed,  and  the  diaphragm  is 
left  free,  stretching  across  between  the  thoracic  and  ab- 
dominal cavities.  Its  upper  surface  is  brushed  with  a 
soft  pencil  moistened  with  water,  so  as  to  remove  the  sur- 


128  NORMAL  HISTOLOGY. 

face  endothelium  and  allow  the  silver  to  penetrate  readily 
into  the  lymphatics.  The  surface  is  now  carefully  washed, 
and  a  one-half-per  cent,  solution  of  nitrate  of  silver  is  al- 
lowed to  run  over  it  and  remain  for  five  minutes  in  con- 
tact with  it.  This  is  now  washed  off  with  water,  the  dia- 
phragm carefully  cut  out  and  pinned  to  a  cork,  so  as  to 
lie  flat,  and  exposed  to  the  light  in  dilute  alcohol  till  it 
becomes  brown.  Small  pieces  are  then  cut  out  from  the 
tendinous  portion  and  mounted  in  glycerin.  In  such  a 
specimen  the  large  lymphatic  channels  will  be  readily 
recognized,  lined  with  elongated  endothelium  like  that  of 
the  blood-vessels,  and  furnished  with  valves,  which  appear 
here  and  there  as  curved  lines  crossing  the  vessel.  The 
capillaries  may  be  recognized  by  their  irregular  shape  and 
sinuous  endothelium. 


CHAPTER  IX. 

LYMPH-NODES— SPLEEN. 
LYMPHATIC    GLANDS    OR    LYMPH-NODES. 

If  we  follow  the  lymphatic  vessels  in  their  course 
from  the  periphery  toward  the  thoracic  duct,  we 
find  that,  sooner  or  later,  they  are  interrupted  in 
their  course  by  certain  nodular  masses,  more 
abundant  in  some  parts  of  the  body  than  in  others, 
and  of  variable  size,  commonly  called  lymphatic 
glands.  They  are  not  glands  in  the  limited  sense  of 
the  word,  for,  so  far  as  we  know,  they  furnish  no 
specific  secretion,  they  have  no  excretory  ducts,  and 
seem  to  have  an  entirely  different  structure  and 
function  from  the  glands  proper.  It  would  be 
better  to  call  them  lymph-nodes,  for  that  would  not 
be  misleading,  as  the  word  gland  is,  in  regard  to 
their  relations  to  other  structures  ;  but,  as  they  are 
universally  known  as  lymphatic  glands,  we  shall  at 
present  use  both  terms.  The  lymph-nodes  present 
a  great  diversity  in  form,  being  spherical,  ovoid, 
discoidal,  or  irregularly  prismatic  ;  they  always 
present  at  one  side  a  hilus,  at  which  the  larger 
blood-vessels  enter. 

If  we  make  a  section  through   a    fresh  gland,  at 

T2n 


I30  NORMAL  HISTOLOGY. 

right  angles  to  its  long  axis  and  through  the  hilus, 
we  find  it  more  or  less  distinctly  divided  into  two 
zones  :  an  outer,  or  cortical  zone — the  cortex — which 
is  soft  and  grayish  or  reddish  in  color,  and  divided 
into  ovoidal  or  irregular-shaped  masses ;  and  an 
inner,  or  medullary  zone — the  medulla — adjacent  to 
the  hilus,  which  is  firmer,  and  has  a  more  uniform, 
or  sometimes  irregularly  reticulated,  grayish,  or 
brownish-red  surface. 

The  nodes  are  surrounded  by  a  firm,  dense  capsule 
of  connective  tissue,  with  a  few  elastic  fibres  and 
smooth  muscle-cells.  The  capsule  sends  inward 
numerous  partitions  or  trabecular,  which  divide  the 
cortex  into  a  series  of  intercommunicating  cham- 
bers, and  the  medulla  into  numerous  irregular  con- 
necting passages.  These  septa  are  formed  of  the 
same  elements  as  the  capsule,  and  at  the  hilus  are 
continuous  with  a  dense  mass  of  connective  tissue, 
through  which  the  blood-vessels  enter  the  organ.  If 
we  examine  the  spaces  left  between  the  septa,  we 
find  that  in  the  cortex  they  are  incompletely  filled 
with  ovoidal  or  globular  bodies,  which  are  continu- 
ous with  cord-like  anastomosing  structures  lying  in 
the  narrower  and  irregular  spaces  in  the  medulla  ; 
the  bodies  in  the  cortex  are  called  lymph-follicles, 
and  the  cord-like  structures  in  the  medulla,  lymph- 
cords. 

If,  now,  we  examine  more  minutely  the  structure 
of  the  lymph-follicles,  we  find  that   they  consist,  in 


L  i TMPII-NODES.  I  3 1 

the  first  place,  of  a  framework  of  reticular  con- 
nective tissue,  whose  meshes  are  largest  in  the 
centre  of  the  follicle,  narrower  and  smaller  in  the 
periphery  ;  indeed,  so  closely  crowded  together  are 
the  trabecular  here,  that  they  give  to  the  follicle  a 
tolerably  well-defined  outline.  In  the  second  place, 
the  meshes  of  the  reticulum  are  closely  filled  with 
small  spheroidal  cells,  having,  in  general,  the  char- 
acters of  lymph-cells  ;  in  many  of  them,  however, 
the  nucleus  is  very  large,  occupying  the  greater 
part  of  the  cell.  The  follicles  do  not  entirely  fill 
the  cavities  in  which  they  lie,  but  are  surrounded 
on  all  sides  by  a  narrow  space. 

If  we  examine  the  relation  of  the  follicles  to  the 
walls  of  their  investing  spaces,  we  find  that,  from 
the  walls  of  these  spaces,  delicate  branching  trabec- 
ular pass  inward  to  the  surface  of  the  follicle, 
where  they  become  continuous  with  the  reticular 
tissue  of  the  latter.  They  are,  in  fact,  themselves 
reticular  connective  tissue,  similar  to  that  of  the 
follicle,  except  that  the  trabecular  are  coarser  and 
the  meshes  broader.  Stretching  across  the  space 
surrounding  the  follicles,  they  suspend  the  latter  so 
that  they  hang  free  in  the  cavities.  The  space 
thus  formed  around  the  follicles  is  called  the  peri- 
follicular space,  or,  better,  lymph-sinus,  for  reasons 
which  will  be  presently  given. 

If  we  now  turn  our  attention  to  the  lymph-cords 
of  the  medulla,  which,  it   will   be   remembered,  are 


132  NORMAL  HISTOLOGY. 

continuous  with  the  follicles,  we  find  that  they  have 
an  exacth'  similar  structure,  and  bear  the  same 
relation  to  the  more  irregularly  arranged  connective- 
tissue  septa  which  bound  the  branching  spaces  in 
which  they  ramify,  that  the  follicles  do  to  the  walls 
of  their  investing  spaces,  i.  c,  they  are  suspended 
in  them  by  coarse  trabecular  of  reticular  connective 
tissue,  and  surrounded  on  all  sides  by  lymph- 
sinuses.  These  lymph-cords,  ramifying  and  inoscu- 
lating in  the  medullary  portion  of  the  gland,  form 
an  intricate  svstem  of  intercommunications  between 
all  the  follicles  of  the  gland.  Injections  of  dilute 
solutions  of  nitrate  of  silver  into  the  lymph-sinuses 
show  that  the  surfaces  of  the  follicles  and  lymph- 
cords,  as  well  as  the  walls  of  their  investing  spaces, 
are  covered  with  endothelium. 

If  we  study  the  relation  of  the  lymph-vessels  to 
the  lymph-nodes,  we  find  that  the  former,  on  arriv- 
ing at  the  surface  of  the  node — afferent  vessels — 
pierce  the  capsule  and  become  continuous  with  the 
lymph-sinuses,  into  which  they  pour  their  contents; 
we  find,  further,  that  efferent  vessels,  still  continuous 
with  the  sinuses,  leave  the  organ  at  other  points, 
frequently  at  the  hilus.  The  lymph-glands,  then, 
are  structures  interrupting  the  course  of  the  lymph- 
atic vessels,  in  which  the  lymph  is  forced  to  pass 
through  a  series  of  irregular-branching  spaces  or 
sinuses,  bathing  in  its  course  certain  peculiar  struc- 
tures— the  follicles  and  lymph-cords — -whose  function 


LYMPH-NODES.  133 

we  do  not  yet  understand.  The  lymph-sinuses  con- 
tain not  only  the  fluid  of  the  lymph,  but  numerous 
lymph-cells,  and  usually  a  certain  number  of  red 
blood-cells. 

The  principal  blood-vessels  enter  the  nodes  at  the 
hilus,  and  the  arteries,  sending-  off  branches  to  the 
connective  tissue  there  and  to  the  septa,  divide  and 
subdivide,  and  soon  enter  the  lymph-cords ;  they 
pass  along  in  the  axis  of  these,  giving  off  a  long- 
meshed  capillary  system  ;  then  entering  the  follicles, 
they  break  up  into  a  loose  capillary  net-work,  from 
which  the  blood  is  collected  into  venous  radicles, 
and  poured  into  veins  which  pass  out  through  the 
lymph-cords  and  out  of  the  organ  in  connection  with 
the  arteries.  Small  blood-vessels  usually  enter  the 
capsule  at  other  points  than  the  hilus,  and  are 
largely  distributed  to  the  capsule  and  the  connective 
tissue  of  the  septa. 

The  number  and  arrangement  of  the  follicles 
vary  greatly  in  different  lymphatic  glands :  in  some 
there  is  but  a  single  layer  in  the  cortex ;  in  others, 
several  layers  are  superimposed  and  more  or  less 
crowded — thus  giving  to  some  glands  a  narrow,  to 
others  a  broad  and  voluminous  cortex.  In  many 
animals,  as  the  ox,  the  reticular  tissue  of  the  me- 
dullary portion  contains  an  abundance  of  brown 
pigment. 

There  are,  in  many  parts  of  the  body,  small  dense 
masses  of  tissue,  some  of  them  sharply  circumscribed, 


134  NORMAL  HISTOLOGY. 

others   diffuse   and   merging  into   adjacent    tissues, 
which  seem  to  be  somewhat  analogous  to  the  lymph- 
nodes,  although   not,  as  a  rule,  forming  well-defined 
organs.     Thus  we  find  in  the  intestines  and  stomach, 
either  single  or  in  clusters,  circumscribed  nodules  of 
reticular  connective  tissue,  whose  meshes  are  filled 
with  small  spheroidal  cells,  and  resembling  in  most 
respects   the   follicles   in  the   cortex  of  the  lymph- 
nodes.     These,  which  will  be  more  fully  considered 
when  we  study  the  gastro-intestinal  canal,  are  called 
the  solitary  follicles  of  the  stomach  and  intestines, 
and  Peyer's  patches.     Less  well  defined  than  these, 
we  find  scattered  in  various  parts  of  the  body,  larger 
and  smaller  diffuse  collections  of  small   spheroidal 
cells,  lying  in  a  reticular  stroma,  and  forming  the  so- 
called  lymphoid  tissue,  which  recent  investigations 
have  shown  to  be  of  no  little  importance  under  cer- 
tain pathological  conditions,  although  of  their  rela- 
tions to  the  lymph-vessels  or  their  physiological  sig- 
nificance we  know  very  little.     These  diffuse  collec- 
tions of  lymphoid  tissue  are  found  in  the  mucous 
membrane   of   the  bronchi,  beneath   certain  serous 
membranes,  in  the  liver,  kidneys,  and  elsewhere,  and 
may  be  seen  in  the  preparations  of  these  parts  pres- 
ently to  be  studied. 


PRACTICAL    STUDY. 


Lymph-Sinuses  Injected. — A  general  view  of  the  lymph- 
sinuses  and  their  relations  to  the  follicles  and  cords  is 


L  YMPH-NODES.  I  3  5 

best  obtained  from  sections  of  nodes  whose  lymph-channels 
have  been  filled  with  a  colored  solution  of  gelatin.  A 
gland  being  exposed  in  a  recently  killed  animal — one  of 
the  cervical  glands  of  the  dog  answers  well — a  hypodermic 
syringe  is  warmed  and  filled  with  the  warmed  blue 
gelatin  mixture  ;  the  canula  is  now  thrust  through  the 
capsule  at  any  point,  and  the  fluid  injected.  The  node 
will  become  mottled  with  blue,  and  the  mass  will  often  be 
seen  to  flow  into  adjacent  nodes.  If  it  be  not  desired  to 
inject  more  than  one,  a  ligature  should  be  passed  around 
the  vessels  leading  to  the  others.  When  a  sufficient 
quantity  of  the  fluid  has  been  injected  to  render  the  gland 
firm  and  the  capsule  tense,  the  canula  is  withdrawn,  and 
the  node  cooled  by  ice  or  cold  water.  When  the  gelatin 
has  solidified,  the  gland  is  cut  out,  divided  longitudinally, 
and  put  into  strong  alcohol.  When  it  has  become  suffi- 
ciently hard,  sections  are  made  through  the  entire  gland, 
stained  with  picro-carmine  or  eosin,  and  mounted  in 
balsam. 

Shakefi  Section  of  a  Lymph-node. — The  meshes  of  the 
follicles  and  cords,  as  well  as  the  lymph-sinuses,  are  so 
closely  crowded  with  cells,  that  the  structure  of  the  sup- 
porting tissue  cannot  be  clearly  made  out,  even  in  very 
thin  sections,  without  removing  some  of  the  cells.  This 
may  be  done  by  shaking  in  water,  as  in  the  preparation 
of  reticular  connective  tissue  p. 5 5).  When  the  operation 
is  completed — which  will  be  determined  by  the  extreme 
thinness  which  the  section  presents  when  most  of  the  loose 
cells  are  removed — it  is  floated,  without  folds,  on  to  a 
slide,  stained  double  and  mounted  in  glycerin. 

Blood-vessels. — To  obtain    an   injection   of   the  blood- 


136  NORMAL  HISTOLOGY. 

vessels  of  the  lymph-nodes,  either  a  whole  animal,  such 
as  the  rabbit  or  dog,  may  be  injected  through  the  aorta, 
with  the  blue  gelatin  mixture  :  or  a  single  gland,  such  as 
the  cervical  or  mesenteric,  may  be  injected  through  its 
main  artery.  The  glands  should  be  hardened  in  alcohol, 
and  the  sections  stained  deeply  with  eosin  and  mounted 
in  balsam. 

THE     SPLEEN". 

The  spleen,  although  differing  in  many  important 
and  probably  most  essential  particulars  from  the 
lymphatic  glands,  yet  presents  many  striking  analo- 
gies with  them.  Like  them,  it  presents,  on  cross- 
section,  to  the  naked  eye.  a  fibrous  envelope — the 
capsule, — from  which  septa  and  trabecular  pass  into 
the  organ,  enclosing  irregular  spaces.  Here  also  we 
find  the  spaces  between  the  septa  filled  with  a  soft 
substance  presenting  two  distinct  modes  of  arrange- 
ment ;  we  find  first,  irregularly  scattered  through  the 
organ,  small  grayish  globular  or  elongated  struc- 
tures, called  Malpighian  bodies,  or  follicles ;  and,  sec- 
ond, between  these,  filling  up  the  remaining  space 
between  the  trabecular,  a  soft  red  tissue  called  the 
pulp. 

Finally,  we  find  blood-vessels  entering  the  organ 
at  the  hilus.  We  have,  then,  in  studying  the 
spleen,  to  consider  the  connective-tissue  capsule  and 
trabecules,  the  follicles,  the  pulp,  and  the  blood- 
:  essels. 

The  capsule  of   the  spleen  consists  of  a  dense  en- 


THE  SPLEEN.  137 

vclope   of  interlacing   connective-tissue   fibres   with 
flattened   cells,  with  a  large  number  of  fine  clastic 
fibres  and  a  few  smooth  muscle-cells.     It  is  covered 
by  a  layer  of  endothelial  cells  similar  to  those  of  the 
general  peritoneal   surface.     At  the   hilus  it  passes 
inward,  forming  a  sheath   for  the  large  vessels,  and 
joins  a  complicated  system   of  septa  and  trabecule?, 
which,  proceeding  inward  from  all  parts  of  the  cap- 
sule, form  a  multitude   of   irregular  communicating 
spaces  in  which  the  follicles  and   splenic   pulp  lie. 
These  trabecular  and  septa  are  made  up  of  the  same 
elements  as  the  capsule,  and  in  size  and   abundance 
vary  greatly  in  different  animals,  being  in  man  only 
moderately  developed.     The  follicles,  or  Malpighian 
bodies,  have  essentially  the  same  structure  as  the  fol- 
licles of  the  lymph-nodes — that   is,  they  are   formed 
by  a  small  mass  of  supporting  reticular  connective- 
tissue,  whose  meshes  are  narrowest  at  the  periphery, 
and  closely  filled  throughout  with    small    spheroidal 
cells,  and   supplied  with    a   net-work   of  capillaries  ; 
we  usually  find  here,  however,  a  small  artery  passing 
either  through  the  centre   or   at  one  side  of  the  fol- 
licle. 

But  in  order  to  fully  understand  the  follicles-  of 
the  spleen,  it  is  necessary  to  study  their  relation  to 
the  arteries,  in  which  respect  they  seem  entirely  to 
differ  from  their  analogues  in  the  lymph-nodes. 
The  arteries  and  veins  as  they  enter  the  hilus  of  the 
spleen  are  surrounded,   as  above   mentioned,   by  a 


I3S  NORMAL  HISTOLOGY. 

connective-tissue  sheath  ;  after  passing  for  a  short 
distance  inward,  they  separate,  and  the  arteries,  still 
accompanied  by  a  certain  amount  of  connective 
tissue,  divide  and  subdivide,  proceeding  farther  in- 
ward, until  the  small  branches  finally  break  up  into 
brush-like  bundles  of  delicate  twigs.  If,  now,  we 
carefully  study  the  walls  of  the  smaller  arteries,  we 
find  that  in  certain  parts  they  undergo  a  singular 
modification  :  at  first  the  connective-tissue  sheath 
and  the  adventitia  become  very  loose  in  texture,  and 
their  meshes  become  rilled  with  spheroidal  cells  re- 
sembling lymph-cells — this  is  called  lymphoid  infil- 
tration of  the  walls  of  the  arteries ;  then  we  find  that 
at  certain  points  this  infiltration  becomes  quite  ex- 
tensive, the  intercellular  substance  assuming  the 
character  of  reticular  connective  tissue  ;  and  thus 
distinct  spheroidal  or  much  elongated  swellings  are 
formed  either  around  or  at  one  side  of  the  arteries 
— these  are  the  splenic  follicles  or  Malpighian  bodies. 
In  some  animals  this  infiltration  is  quite  extensive 
and  continues  along  the  arteries  for  a  considerable 
distance  at  either  side  of  the  follicles  ;  in  others,  as 
in  man,  it  is  not  very  marked  except  in  the  follicles, 
but  ma}'  frequently  be  seen  along  the  arteries  ad- 
jacent to  them,  in  the  form  of  narrow  cellular 
sheaths.  The  capillary  net-work  of  the  follicles  is 
connected  with  arterial  twigs  which  either  penetrate 
from  without,  or  are  given  off  from  the  follicular 
artery  as  it  passes  through  the  body. 


THE  SPLEEN.  I  39 

Let  us  now  turn  to  the  pulp.  This  is  composed, 
in  the  first  place,  of  a  multitude  of  irregular,  fre- 
quently anastomosing  cords — called  pulp-cords — 
between  which,  and  bounded  closely  by  them,  lie,  in 
the  second  place,  a  series  of  branching  channels — 
the  cavernous  veins.  The  pulp-cords — joined  on  the 
one  hand  to  the  follicles  and  infiltrated  arterial 
sheaths,  and  on  the  other  to  the  connective-tissue 
septa — consist  of  a  framework  of  delicate  reticular 
connective  tissue,  continuous  with  the  sustaining 
tissue  of  the  follicles,  the  meshes  of  which  are  in- 
completely filled  with  various  kinds  of  cells.  Among 
these  cells  we  find  spheroidal  cells,  like  lymph-cells ; 
larger  colorless  cells  with  one  or  more  large  nuclei ; 
red  blood-cells  ;  fragments  of  red  blood-cells  ;  larger 
and  smaller  colorless  cells  containing  pigment  in 
various  forms.  Finally,  there  are  sometimes  found 
in  varying  number,  cells  which  resemble  the  lymph- 
and  larger  colorless  cells  in  form,  but  whose  bodies, 
either  homogeneous  or  granular,  have  a  color  sim- 
ilar to  that  of  the  red  blood-cells.  These  latter 
cells,  like  certain  similar  cells  already  mentioned  as 
occurring  in  the  marrow  of  bones,  are  called  nu- 
cleated red  blood-cells,  and  are  regarded  by  many 
observers  as  intermediate  forms  between  the  color- 
less and  the  red  blood-cells.  Although  many  re- 
cent observations  would  tend  to  confirm  this  idea, 
their  nature  is  as  yet  by  no  means  absolutely  de- 
termined. 


14-0  NORMAL  HISTOLOGY. 

We  have  still  to  consider  the  structure  of 
the  second  constituent  of  the  pulp — the  cavernous 
veins.  Following  the  splenic  veins  inward  from 
the  hilus,  we  find  that  they  gradually  lose 
their  connective-tissue  sheath,  and  then  their 
outer  coats,  and  then  rapidly  divide  and  sub- 
divide to  form  a  multitude  of  intercommunica- 
ting thin-walled  canals  of  tolerably  uniform  calibre, 
which  occupy  the  irregular-branching  spaces  be- 
tween the  pulp-cords.  These  ultimate  venous  trunks 
are  called  cavernous  veins,  and  their  walls  consist  of 
little  else  than  a  few  widely  separated,  branching, 
circular  and  oblique  fibres,  upon  which  lie,  at  vary- 
ing intervals,  elongated,  curved,  spindle-shaped,  and 
flattened  endothelial  cells,  which  have  their  long 
axes  parallel  with  the  axis  of  the  canal.  These  veins, 
whose  walls,  as  will  be  seen,  are  not  closed  but  fenes- 
trated, are  in  direct  communication  with  the  spaces 
which  are  still  left  in  the  meshes  of  the  pulp-cords 
by  the  cells  which  incompletely  fill  them. 

Still  another  point  remains  to  be  considered, 
namely,  the  course  of  the  blood  after  its  exit  from 
the  above-described  fine  arterial  twigs  on  which  the 
Malpighian  bodies  are  formed,  until  it  enters  the 
fenestrated  cavernous  veins  of  the  pulp.  The 
opinion  of  different  observers  on  this  point  differs 
somewhat,  owing  to  the  extreme  technical  difficul- 
ties in  the  investigation  ;  but  it  seems  probable  that 
after  passing  out  of  the  fine  arterial  twigs,  through 


THE  SPLEEN.  141 

the  intervention  of  the  capillaries  it  is  poured  direct- 
ly into  the  meshes  of  the  pulp-cords,  and  that  after 
circulating  here  around  the  cells,  without  distinctly 
walled  channels \  it  finally  finds  its  way  through  their 
fenestrated  walls  into  the  cavernous  veins,  whence 
it  passes  out  of  the  organ  through  the  large  efferent 
veins  at  the  hilus. 

PRACTICAL   STUDY. 

Sections  of  Uninjected  Spleen,  a.  Cat. — A  general 
view  of  the  arrangement  of  the  different  structures  of  the 
spleen  may  be  obtained  from  very  thin  sections  of  a  cat's 
spleen  hardened  in  bichromate  of  potassium  and  alco- 
hol. They  are  stained  double  and  mounted  in  bal- 
sam, b.  Human. — Structural  details  may  be  studied  in  a 
section  from  a  human  spleen  hardened  as  above.  Before 
mounting,  the  sections  should  be  placed  in  a  shallow,  flat- 
bottomed  dish,  and  just  covered  with  water  ;  with  a  fine 
camel's  hair  pencil,  held  perpendicularly,  the  section  is 
gently  tapped  until  it  appears  thinner  and  more  transpa- 
rent from  the  brushing  out  of  the  loose  cells.  It  may 
then  be  stained  double,  and  mounted  in  glycerin  or  bal- 
sam. 

Section  of  Spleen  with  Injected  Cavernous  Veins. — A 
spleen  is  injected,  under  low  pressure,  through  the  vein, 
with  the  blue  gelatin  mixture,  hardened  in  alcohol,  stained 
deeply  with  eosin,  and  mounted  in  balsam. 

Isolated  Cells  of  the  Spleen. — A  bit  of  the  fresh  organ  is 
teased  on  a  slide  in  a  half-per-cent.  salt  solution,  and 
studied   in    the   same.      Should    the   red  blood-cells,  ns  is 


H2  NORMAL  HISTOLOGY. 

apt  to  be  the  case,  be  so  numerous  as  to  conceal  the 
other  forms,  a  drop  or  two  of  dilute  alcohol  (one  to 
two)  is  allowed  to  run  under  the  cover-glass,  and  after  a 
few  moments  the  red  blood-cells  will  have  become  so 
transparent  as  not  seriously  to  interfere  with  the  observa- 
tion. If  the  nuclei  are  not  sufficiently,  well  defined,  a 
drop  of  eosin  may  be  added  after  the  alcohol.  In  addi- 
tion to  the  above-described  cells  of  the  pulp-cords  in 
teased  specimens,  narrow,  elongated,  often  curved  cells, 
with  projecting  nuclei,  are  frequently  seen  ;  these  are  the 
above-described  lining  cells  of  the  cavernous  veins. 


CHAPTER  X. 
THE  GASTROINTESTINAL   CANAL. 

THIS  canal  is  a  tube  varying  greatly  in  its  calibre 
in  different  parts,  and  continuous  at  either  end  with 
the  external  surface  of  the  body.  In  certain  part?. 
of  its  course  it  is  intimately  connected  with  adja- 
cent structures ;  but,  for  the  most  part,  it  is  attached 
only  at  one  side  by  a  structure — the  mesentery — 
which  serves  to  convey  to  it  its  blood-  and  lym- 
phatic-vessels and  nerves.  The  walls  of  the  tube, 
although  varying  in  structure  in  different  sections, 
consist  in  general  of  a  muscular  layer,  a  mucous 
layer  lining  the  tube,  and,  in  those  parts  where 
it  is  suspended  in  the  abdominal  cavity,  a  serous 
layer  covering  it. 

Confining  our  attention  to  the  stomach  and  in- 
testines, we  find  that  these  layers  are  not  simple, 
but  have  each  a  composite  structure  ;  thus,  we  find 
in  the  serosa,  a  layer  consisting  chiefly  of  dense 
fibrillar  connective  tissue,  subserosa,  covered  with  a 
layer  of  endothelium.  The  muscular  tunic,  or  mus- 
culosa,  consists  of  two  layers  of  smooth  muscular 
tissue:  an  external,  in  which    the  cells  lie  longitudi- 


144  NORMAL  HISTOLOGY. 

nally,  and  an  internal  in  which  they  lie  transversely 
to  the  axis  of  the  canal.  In  certain  parts  of  the 
stomach  an  indistinct  third  layer  is  found,  in  which 
the  cells  have  an  oblique  course.  Inside  of  the 
muscularis  and  joining  it  to  the  mucosa,  is  a  layer  of 
loose  fibrillar  connective  tissue,  called  the  snbmucosa, 
in  which  the  blood  and  lymphatic  vessels  ramify. 
In  the  mucosa,  finally,  we  have  a  delicate  support- 
ing framework  of  connective  tissue,  varying  some- 
what in  its  structure  and  abundance  in  different 
parts  of  the  canal  ;  this  is  covered  by  epithelial 
cells  and  contains  the  glandular  apparatus,  while  at 
the  base  of  the  glands  and  adjacent  to  the  submu- 
cosa  is  a  thin  layer  of  smooth  muscle-cells,  lying  in 
both  transverse  and  oblique  directions,  called  the 
muscularis  mucosa,  from  which  usually  a  few  muscle- 
cells  pass  up  between  the  glands. 

The  chief  differences  in  minute  structure  between 
the  stomach  and  intestines  are  in  the  mucous  mem- 
brane, and  since  in  this  the  glands  are  very  impor- 
tant factors,  a  word  should  be  said  here  about  the 
structure  of  glands  in  general. 

Although  the  term  gland  is  popularly  applied  to 
structures  having  the  greatest  diversity  of  form  and 
function,  and  little  in  common  but  their  name,  we 
mean  by  it  here,  those  organs  whose  physiological 
activity  expresses  itself,  in  part  at  least,  by  the 
elaboration  of  certain  specific  fluids,  secretions,  or 
excretions.    All  such  elands  have  a  somewhat  analo- 


THE  CASTRO-INTESTINAL  CANAL.  14$ 

gous  structure,  and  present  two  distinct  kinds  of 
structural  elements :  1.  epithelial  or  gland-cells;  2. 
a  connective-tissue  framework,  with  blood-  and 
lymphatic-vessels  and  nerves.  The  epithelial  cells, 
usually  large,  differ  in  form  in  different  and  even  in 
the  same  glands ;  in  the  latter  case  especially,  when 
the  gland  is  divided  into  a  secreting  and  excretory 
portion.  They  will  be  described  when  we  study  the 
glands  in  detail.  The  connective  tissue,  varying 
greatly  in  amount  in  different  glands,  is  sometimes 
arranged  in  sheets  and  bundles  so  as  to  form 
variously  shaped  cavities  which  are  lined  with  the 
gland-epithelium  ;  sometimes,  in  the  form  of  simple 
or  superimposed  lamellae  covered  with  flat  cells  like 
endothelium,  it  forms  thin-walled  tubes  or  chambers 
on  whose  sides  the  cells  are  placed  ;  in  this  form  it 
is  called  the  mcmbr ana  propria  of  the  gland  cavities. 
The  cavities  and  tubes  thus  formed  and  lined  with 
gland-epithelium  are  variously  arranged  in  different 
glands,  but  their  different  modes  of  arrangement 
may  be  reduced  to  three  types : 

1.  Tubular  glands,  which  have  the  form  of  simple 
or  occasionally  branching,  straight,  curved,  or  vari- 
ously contorted  tubes,  terminating  in  blind  extremi- 
ties.    Such  are  the  glands  of  the  stomach. 

2.  Racemose  glands,  in  which  the  secreting  portion 
in  the  form  of  vesicular  or  irregular-shaped  cavities 
— alveoli — are  grouped  around  simple  or  branching 
excretory  ducts,  into  which  they  open  ;    the  whole 


I46  NORMAL  HISTOLOGY. 

structure  has  been  not  inaptly  compared  to  a  bunch 
of  grapes,  in  which  the  fruit  would  correspond  to 
the  alveoli,  the  stem  to  the  excretory  ducts  ;  the 
analogy  failing  at  this  point,  however,  for  in  the 
gland  the  alveoli  and  ducts  are  bound  together  by 
connective  tissue  lying  between  them — called  inter- 
stitial tissue — in  which  the  vessels  and  nerves  ramify. 
The  alveoli  which  open  into  the  same  excretory 
duct  are  usually  joined  more  closely  to  one  another 
than  to  those  opening  into  different  ducts,  and  these 
clusters  of  alveoli  are  called  the  lobules  or  acini  of 
the  gland.  Such  are  the  mammary  glands  and  cer- 
tain mucous  glands  of  the  bronchi. 

3.  Vesicular  Glands. — These  consist  of  simple, 
spheroidal,  or  irregular-shaped  closed  alveoli,  sur- 
rounded by  a  membrana  propria,  and  lined  with 
epithelium,  the  separate  alveoli  being  embedded  in 
interstitial  connective  tissue.  Such  glands  are  the 
thyroid*  and  ovary. 

THE    STOMACH. 

The  muscularis  of  the  stomach  differs  from  that 
of  the  intestines,  in  that  a  certain  number  of  the 
cells,  especially  in  the  cardiac  extremity,  do  not 
have  the  typical  transverse  or  longitudinal  arrange- 
ment, but  lie  in  an  oblique  direction.  In  the  vicinity 
of  the  pylorus  again,  the  inner  circular  layers  are 
much  thickened,  forming  the  sphincter  pylori.  The 
mucosa  is  entirely   made   up   of   glands  supported 


THE  G 'ASTRO-INTESTINAL  CANAL.  1 47 

and  held  together  by  a  small  amount  of  delicate 
connective  tissue,  in  which  the  blood  and  lymphatic 
vessels  ramify.  The  glands  are  tubular,  sometimes 
simple,  sometimes  divided,  and  often  tortuous  at 
the  base.  They  have  a  membrana  propria,  and,  de- 
pending upon  differences  in  the  epithelium  which 
line  them,  they  are  classified  as:  I.  mucous  glands ; 
2.  peptic  glands. 

The  general  surface  of  the  stomach  is  covered 
with  cylindrical  epithelium.  In  the  pyloric  region, 
and  here  and  there  in  other  parts,  the  glands,  or 
follicles,  as  they  are  often  called,  are  lined  through- 
out with  cylindrical  epithelium  ;  these  are  the  mu- 
cous glands. 

The  greater  proportion  of  the  glands,  however, 
are  lined  only  at  their  orifices  with  cylindrical  epi- 
thelium ;  deeper  down  in  the  gland  we  find  usually 
two  kinds  of  cells :  a,  spheroidal  or  polyhedral  cells, 
with  transparent  or  very  finely  granular  bodies; 
and  b,  larger  spheroidal,  or  somewhat  flattened,  very 
granular  cells,  which  usually  lie  outside  the  others, 
between  them  and  the  membrana  propria.  These 
are  the  so-called  peptic  cells,  and  these  glands  are 
called  peptic  glands.  The  relative  number  of  these 
different  kinds  of  peptic  cells  varies,  depending  upon 
the  degree  of  functional  activity  of  the  glands. 
When  they  are  secreting  rapidly,  the  granular  cells 
are  abundant  ;  when  at  rest  they  are  few  in  number, 
the  smaller  transparent  cells  preponderating. 


I48  NORMAL  HISTOLOGY. 

The  arteries  pass  obliquely  through  the  serosa 
and  musculosa,  divide  and  subdivide  in  the  loose 
tissue  of  the  submucosa,  from  whence  branches  are 
sent  in  between  the  glands  ;  here,  before  reaching 
the  surface,  they  break  up  into  a  close  capillary  net 
surrounding  the  follicles,  and  the  blood  is  finally 
collected  into  narrow  venous  trunks  directly  be- 
neath the  surface  epithelium  ;  from  these  it  passes 
back  into  larger  veins  in  the  submucosa,  where  it 
collects  in  the  efferent  veins.  The  lymphatic  ves- 
sels lie  between  the  glands,  form  anastomosing 
channels  in  the  submucosa,  and  pass  out  through 
the  musculosa,  receiving  larger  and  smaller  trunks 
from  the  latter. 

The  nerve-trunks  from  the  sympathetic  and  pneu- 
mogastric  form  a  plexus,  associated  with  minute 
ganglia,  called  Anerbactis plexus,  between  the  layers 
of  the  musculosa;  from  this  branches  pass  into  the 
submucosa  and  form  another  similar  plexus,  called 
Meissner  s  plexus. 

Nodules  of  lymphoid  tissue,  varying  greatly  in 
size  and  number,  are  found  in  the  mucosa  of  the 
stomach,  at  the  base  of  the  follicles,  and  sometimes 
extending  up  between  them.  These  are  sometimes 
visible  to  the  naked  eye  as  small  grayish  promi- 
nences on  the  surface  of  the  mucous  membrane,  and 
have  been  called  the  lenticular  glands  or  follicles  of 
the  stomach. 


THE  G ASTRO-INTESTINAL  CANAL.  1 49 

THE    SMALL    INTESTINE. 

The  supporting  connective-tissue  framework  of 
the  mucosa  in  the  small  intestine  is  more  abundant 
than  in  the  stomach,  and  is  richly  infiltrated  with 
small  spheroidal  and  variously  shaped  ceils.  In  it 
lie  embedded  tubular  glands,  not  unlike  the  mucous 
glands  of  the  stomach,  but  not  crowded  so  closely 
together.  These  glands  are  often  called  the  folli- 
cles of  Liebe?'kului,  and  are  lined  with  cylindrical 
epithelium.  Rising  from  the  general  surface  of  the 
mucous  membrane,  between  the  orifices  of  the 
glands,  are  very  numerous  short  cylindrical  or  coni- 
cal projections  called  villi.  These  are  formed  by 
projections  inward  of  the  mucosa ;  they  are  covered 
by  cylindrical  epithelium,  and  contain  an  abundant 
vascular  net-work,  and  the  radicles  of  the  lymph-  or 
chyle-vessels. 

The  cylindrical  epithelial  cells  are  joined  to- 
gether, side  by  side,  by  cement  substance,  and  pos- 
sess a  marked  peculiarity  in  the  structure  of  the 
free  border,  which  is  considerably  thickened,  and  is 
crossed,  in  a  direction  corresponding  with  the  long 
axis  of  the  cell,  by  fine  parallel,  closely-set  lines, 
which  are  usually  interpreted  as  tiny  pores  or  canals 
passing  through  the  border.  It  was  formerly  taught 
that  through  these  pores  the  chyle  passed  to  enter 
the  epithelium  on  its  way  to  the  lymph-vessels. 
More    recently,    however,    the    view    has    been    ad- 


150  NORMAL  HISTOLOGY. 

vanced  that  substances  absorbed  into  the  lymph- 
vessels  from  the  intestines  pass,  not  through  the 
epithelial  cells,  but  through  the  cement  substance 
between  them. 

Scattered  here  and  there  between  the  cylindrical 
epithelium,  sometimes  abundant,  sometimes  not, 
are  transparent  more  or  less  ovoidal  cells  with  a 
nucleus  and  a  small  amount  of  protoplasm  in  the 
vicinity  of  the  narrow  base  ;  they  look  as  if  the 
free  border  of  the  cell  had  fallen  off  and  most  of 
the  cell-contents  had  disappeared.  Not  infre- 
quently a  translucent  structureless  substance  is  seen 
protruding  from  the  open  end  of  the  cell,  as  if  in 
the  act  of  passing  out  of  it.  These  cells  are  called, 
from  their  form,  goblet-cells.  Their  significance  is 
not  yet  definitely  determined  in  all  cases  :  by  some, 
they  are  regarded  as  cylindrical  cells  changed  by 
artificial  means  ;  but  most  observers  believe  them 
to  be  normal  structures,  and  suppose  that  under 
certain  circumstances  the  cell-contents  undergo  a 
mucous  metamorphosis,  swell  up,  burst  out  of  the 
cell,  leaving  little  but  the  membrane  and  nucleus 
behind,  and  that  thus,  under  normal  conditions,  a 
certain  amount  of  the  mucus  furnished  by  mucous 
membranes  is  produced. 

In  addition  to  the  tubular  glands  which  are  found 
throughout  the  whole  extent  of  the  small  intestine 
— in  the  duodenum,  especially  in  its  upper  por- 
tions— racemose,     probably      mucous     glands,    are 


THE  GASTRO-INTESTINAL  CANAL.  1 5  I 

found,  called  Briuincr  s  glands.  They  lie  in  the 
submucosa,  and  consist  of  variously  shaped,  but 
usually  elongated  alveoli,  surrounded  by  a  mem- 
brana  propria  and  lined  with  cylindrical  epithelium. 
The  excretory  ducts  are  also  lined  with  cylindrical 
epithelium,  and  open  on  the  surface  of  the  mucous 
membrane. 

Smooth  muscle-cells  pass  up  from  the  muscularis 
mucosas  into  the  villi.  The  central  portion  of  the 
villi  is  occupied  by  one  or  more  usually  blind 
canals — the  chyle-vessels — which  pass  outward  to 
the  bases  of  the  villi,  where  they  usually  unite  to 
form  a  net-work  around  the  orifices  of  the  tubular 
glands  of  Lieberkiihn  and  the  lymph-follicles  pres- 
ently to  be  described ;  they  then  pass  into  the 
submucosa,  where  they  form  larger  anastomosing 
channels ;  from  thence  trunks  pass  through  the 
musculosa,  receiving  vessels  from  its  two  layers,  and 
from  a  well-developed  net-work  between  them. 
The  distribution  of  the  nerves  is  essentially  similar 
to  that  in  the  stomach. 

Closely  connected  with  the  lymphatic  vessels,  and 
apparently  forming  a  part  of  the  lymphatic  appa- 
ratus of  the  intestines,  are  found  certain  structures 
called  lymphatic  follicles,  and  of  these  it  is  custom- 
ary to  distinguish  two  kinds:  I.  Solitary  follicles ; 
and  2.  Agminatcd  follicles,  or  Peyers  patches. 

1.  Solitary  Follicles. — These  are  irregularly  scat- 
tered through  the  mucous  membrane  of  both   small 


152  NORMAL  HISTOLOGY. 

and  large  intestines,  in  the  form  of  small  grayish 
nodules.  They  lie  chiefly  in  the  mucosa,  often 
piercing  the  mascularis  mucosa  and  descending  into 
the  siibmucosa ;  they  are  usually  spheroidal  or  pear- 
shaped,  and  frequently  project  somewhat  into  the 
intestinal  cavity.  Where  they  lie,  the  tubular 
glands  are  crowded  to  one  side,  and  the  villi  are 
absent  over  their  surfaces.  They  may  lie  so  near 
the  surface  as  to  be  covered  only  by  a  single  layer 
of  cylindrical  epithelium,  or  they  may  be  more 
deeply  placed,  and  covered,  in  addition,  by  a  thin 
layer  of  the  connective  tissue  of  the  mucosa. 
They  consist  of  a  nodule  of  reticular  connective 
tissue,  whose  meshes  are  somewhat  narrower  at  the 
periphery,  where  it  becomes  continuous  with  ad- 
jacent parts.  The  meshes  are  closely  filled  with 
small  spheroidal  cells,  having  the  characters  of 
lymph-cells.  In  their  periphery  the  lymph-vessels 
of  the  mucous  membrane  form  a  closely  anasto- 
mosing net-work.  The  blood-vessels  also  interlace 
in  their  periphery,  and  send  an  abundance  of  anas- 
tomosing capillary  loops  into  their  interior. 

2.  Peyers  Patches. — These  are  found  chiefly  in  the 
small  intestine,  and  here  are  most  abundant  in  the 
lower  portion  of  the  jejunum  and  in  the  ileum  ; 
they  are  round,  or  more  frequently  elongated, 
usually  slightly  elevated  structures,  and  are  always 
situated  at  the  side  opposite  the  mesenteric  attach- 
ment, with  their  long  axes  parallel  with  the  axis  of 


THE  G ASTRO-INTESTINAL  CANAL.  I  53 

the  gut.  They  consist,  essentially,  of  an  aggrega- 
tion of  a  variable  number  of  structures,  having  the 
characters  of  the  solitary  follicles;  these  are  placed 
closely  together,  side  by  side,  and  supplied  in  essen- 
tially the  same  way  as  the  solitary  follicles  with 
blood-  and  lymphatic-vessels. 

THE    LARGE    INTESTINE. 

The  mucosa  of  the  large  intestine  is  thickly  set 
with  tubular  glands  similar  to  Lieberkuhn's  glands 
in  the  small  intestine,,  but  is  destitute  of  villi. 
Solitary  lymphatic  nodules  are  abundant,  and  are, 
as  a  rule,  somewhat  larger  than  those  of  the  small 
intestine.  The  distribution  of  blood-  and  lymphatic- 
vessels  resembles  in  most  respects  that  described  in 
the  stomach  and  small  intestine. 

PRACTICAL   STUDY. 

Sections  of  Stomach. — Bits  of  perfectly  fresh  rabbit's  or 
dog's  stomach,  from  the  fundus  and  the  pyloric  region, 
should  be  stretched  on  a  bit  of  cork  to  prevent  shrink- 
age, and  immersed  in  absolute  alcohol.  After  twenty- 
four  hours  the  fluid  should  be  changed,  and  in  three  or 
four  days  the  specimen  will  probably  be  hard  enough  to 
cut.  After  embedding  in  wax,  sections  from  both 
regions  are  made  perpendicular  to  the  surface,  stained 
double  and  mounted  in  balsam.  The  nuclei  of  all  the 
cells  are  stained  violet  by  the  haematoxylin  ;  in  the  pep- 
tic glands  the  bodies  of  the  granular  peptic  cells  are 
stained  a  deep  rose-red,  while  the  others   are  but  slightly 


154  NORMAL  HISTOLOGY. 

colored,    or    not   at   all.     Sections   perpendicular  to   the 
surface  of  a  stomach,  whose  blood-vessels  are  filled  with 
mixture    of  Prussian-blue    and   gelatin,  and   stained 
with  carmine,  are  very  instructive. 

Sections  of  Intesti?ic. — Bits  of  intestine  from  the  upper 
portion  of  the  duodenum,  from  the  ileum,  including  one 
of  Peyer's  patches,  and  from  the  large  intestine,  shouid 
be  stretched  on  cork  and  immersed  in  a  mixture  of  equal 
parts  of  half-per-cent.  chromic  acid  sol.  and  alcohol  ; 
after  four  days  they  are  transferred  to  alcohol,  in  which, 
in  a  day  or  two,  they  will  become  hard  enough  to  cut. 
Perpendicular  sections  from  the  various  parts  should  be 
stained  and  mounted  in  the  same  way  as  the  stomach. 
Sections  from  intestines,  whose  blood-vessels  are  injected 
with  blue,  are  stained  with  eosin  and  mounted  in  balsam. 


CHAPTER  XL 

SUBMAXILLARY  GLAND— LIVER. 
SUBMAXILLARY    GLAND. 

Connected  with  the  digestive  tract  are  several 
racemose  glands,  which,  although  differing  in  impor- 
tant particulars,  both  in  structure  and  function,  yet 
have  many  features  in  common.  These  glands  are 
the  submaxillar 'is ',  the  sublingual,  the  parotid,  and 
the  pancreas.  Their  details  of  structure  are  still  in- 
sufficiently known,  and  within  the  limits  of  this 
manual  we  cannot  consider  at  length  even  what  is 
well  understood.  We  will  simply  look  at  some  of 
the  more  important  features  of  one  of  the  best 
known,  the  submaxillaris,  considering  this,  in  a  gen- 
eral way  only,  as  typical  of  the  others. 

The  submaxillary  gland  differs  in  structure  in  dif- 
ferent animals,  its  structure  in  the  dog  being  per- 
haps best  known,  and  quite  closely  resembling  that 
in  man.  In  the  dog  it  consists,  like  other  racemose 
glands,  of  alveoli  and  excretory  ducts.  The  elon- 
gated alveoli  are  surrounded  by  a  membrana  propria, 
and  grouped  into  lobules  by  more  or  less  interstitial 
connective  tissue,  which  is  furnished  with  blood-  and 
lymph-vessels  and  nerves.     The  alveoli,  depending 

155 


I  56  NORMAL  HISTOLOGY. 

upon  the  gland-epithelium  which  lines  them,  present 
two  distinct  forms  :  in  011^  form,  the  smaller  of  the 
two,  we  find  the  cavity  of  the  alveoli  nearly  filled 
by  cuboidal  or  polyhedral  cells,  whose  bodies  are 
cloudy  or  granular,  and  which  have  spheroidal  or 
ellipsoidal  nuclei.  In  the  other  form  of  alveoli — ■ 
the  larger — we  find  in  the  first  place,  surrounding 
the  cavity  of  the  alveoli,  large,  irregular-shaped, 
transparent  cells,  having  a  gelatinoid  appearance, 
with  an  often  flattened  nucleus  lying  at  the  periphe- 
ral side  ;  these  cells  are  not  readily  stained  by  eosin, 
and  are  called  mucous  cells.  In  the  second  place,  in 
the  periphery  of  the  alveoli,  between  the  cells  just 
described  and  the  membrana  propria,  lie  large,  in 
cross-section,  crescentic,  strongly  granular  masses, 
usually  containing  several  nuclei ;  these  are  called 
the  crescents  of  Gianuzzi.  They  are  believed  to  be 
formed  by  a  number  of  small  angular  cells  closely 
crowded  together ;  they  are  readily  stained  with 
eosin,  and  are  apparently  analogues  of  the  granular 
peptic  cells  of  the  stomach.  Like  the  latter,  they 
are  most  abundant  when  the  gland  is  in  a  condition 
of  functional  activity,  and  are  believed  to  be  des- 
tined to  replace  the  inner  layer  of  transparent  cells 
as  these  are  destroyed  or  changed  in  burnishing  the 
specific  secretion  of  the  gland. 

The  excretory  ducts  differ  in  structure  and  in  the 
diameter  of  the  lumen,  in  different  parts  of  the 
gland  ;  the  difference  depending  chiefly  upon  differ- 


THE  LIVER.  157 

cnces  in  the  structure  of  the  epithelial  layer  which 
lines  them  ;  this,  in  the  larger  ducts,  is  columnar, 
and  in  the  smaller,  flattened. 

PRACTICAL    STUDY. 

Section  of  Gland  of  Dog, — Small  pieces  of  a  perfectly 
fresh  gland  are  hardened  in  absolute  alcohol.  Very  thin 
sections  are  to  be  stained  with  hematoxylin  and  eosin, 
and  mounted  in  glycerin  or  balsam.  One  frequently 
finds  in  a  gland  taken  from  a  dog  shortly  after  eating, 
certain  groups  of  alveoli  with  the  cells  in  an  active,  others 
with  the  cells  in  a  resting,  condition. 

THE    LIVER. 

The  liver  presents  three  distinct  elements  of 
structure:  1.  The  cellular  elements,  which  in  form, 
function,  and  arrangement,  characterize  the  organ — 
the  liver-cells  or  parenchyma,  2.  The  connective- 
tissue  framework — the  interstitial  tissue,  which  sur- 
rounds the  organ  as  a  capsule,  and  in  its  interior 
supports  the  parenchyma  and  carries  the  larger 
vessels.     3.  The  blood-,  lyntpk-,  3.nd  gall-vessels. 

The  liver-cells  are  large,  have  the  form  of  irregu- 
lar, often  somewhat  elongated  polyhedra  ;  they  have 
a  granular  body  which  frequently  encloses  granules 
of  pigment  and  larger  or  smaller  droplets  of  fat ; 
they  have  one  or  more  vesicular  nuclei  and  nucleoli. 
When  living,  they  are  very  soft,  and  the  isolated 
cells  often  show  depressions  on  their  sides  caused 
by  the  pressure  from  adjacent  blood-vessels. 


I5*5  NORMAL  HISTOLOGY. 

If  we  look  at  the  cut  surface  of  a  fresh  liver  with 
the  naked  eye,  we  find  that  it  presents,  more  or  less 
distinctly,  small  polygonal  or  irregular-shaped 
figures,  which  are  sections  of  certain  groups  of  liver- 
cells,  called  acini  or  lobules.  These  lobules  have,  in 
general,  the  form  of  oblong  polyhedra,  and  the  dif- 
ference in  shape  presented  by  the  sections  is  due  to 
the  fact  that  they  lie  crowded  together,  with  their 
long  axes  lying  in  various  directions.  In  order  to 
understand  the  structure  of  these  lobules,  it  is  neces- 
sary to  study  them  in  connection  with  the  blood- 
vessels of  the  liver,  to  which  they  bear  a  very 
constant  and  characteristic  relation. 

The  liver  receives  its  blood  from  the  portal  vein 
and  the  hepatic  artery  ;  it  is  conveyed  away  by  the 
hepatic  vein.  If  we  follow  the  ramifications  of  the 
hepatic  vein,  we  find  that  it  divides  and  subdivides 
until  it  finally  breaks  up  into  short  terminal  radicles, 
around  which  as  a  centre  the  oblong  liver-lobules 
are  grouped.  From  its  smaller  branches  also,  before 
it  breaks  up  into  terminal  radicles,  small,  short 
branchlets  are  given  off,  which  form  the  centres  of 
lobules.  Veins  bearing  this  relation  to  the  lobules 
are  called  ceiitral  veins,  or  vence  intralobularcs. 

If  now  Ave  follow  the  ramifications  of  the  portal 
vein,  on  the  other  hand,  we  find  that,  dividing  and 
subdividing,  it,  too,  gives  off  small  branchlets  which, 
with  the  terminal  branchlets  into  which  it  finally 
breaks  up,  pass  to  the  surface  of  the  lobules;  there 


THE  LIVER.  159 

they  pour  their  blood  into  a  rich  capillary  net-work 
within  the  lobules,  whence  it  passes  directly  into 
the  radicles  of  the  hepatic  vein  at  the  centre.  The 
capillaries  of  the  lobules  radiate  from  the  central 
vein,  for  the  most  part  nearly  at  right  angles  to  its 
axis,  and  take  more  or  less  direct,  slightly  divergent 
courses  to  the  periphery,  being  connected  with  each 
other  by  frequent  branches.  A  net-work  is  thus 
formed,  in  whose  narrow  and  elongated  meshes  the 
liver-cells  lie,  sometimes  in  a  single  row,  sometimes 
in  several  rows,  depending  upon  the  breadth  of  the 
intercapillary  spaces.  The  central  vein  does  not 
usually  extend  quite  to  the  extremity  of  the  lobules, 
and  here  the  capillaries  are  given  off,  brush-like, 
obliquely  from  its  end.  A  liver-lobule,  then,  is  a 
circumscribed  portion  of  liver-tissue,  having  for  its 
centre  a  brancJdet  of  the  hepatic  vein — vena  intralobu- 
lar is, — and  at  its  periphery  the  terminal  branchlets  of 
the  portal  vein — vencu  interlobulares, — while  between 
these  tzvo  sets  of  vessels,  and  joining  them,  is  a  rich 
capillary  net-zvork,  in  whose  elongated  meshes  lie  rows 
of  liver -cells. 

In  certain  animals,  such  as  the  pig,  the  lobules  are 
very  distinct,  being  surrounded  by  connective  tissue 
which  is  directly  continuous  with  the  connective 
tissue  surrounding  the  larger  trunks  of  the  portal 
vein,  hepatic  artery,  etc.,  and  called  Glisson's  capsule. 
In  this  connective  tissue  between  the  lobules  are 
found,  in  addition   to   the  branchlets  of  the  portal 


100  NORMAL  HISTOLOGY. 

vein,  certain  of  the  terminal  branchlets  of  the 
hepatic  artery  and  the  capillaries  connected  with 
them,  and  the  smaller  gall-ducts.  In  the  human 
liver,  on  the  contrary,  there  is  very  little  connective 
tissue  between  the  lobules,  only  here  and  there  small 
masses  are  seen  surrounding  the  branches  of  the 
portal  vein  and  its  accompanying  vessels,  the  lobules 
merging,  for  the  most  part,  insensibly  into  one 
another.  The  hepatic  artery  sends  its  blood  into 
capillaries  which  are  distributed  largely  to  the  walls 
of  the  vessels  and  the  connective  tissue,  and  it 
finally  passes,  directly  or  indirectly,  into  the  intra- 
lobular capillaries. 

We  have  finally  to  consider  the  gall-passages.  The 
larger  gall-ducts,  lined  with  a  well-developed  mucous 
membrane,  supplied  with  tubular  and  racemose 
mucous  glands,  and  covered  with  cylindrical  epithe- 
lium, enter  the  liver  with  the  other  large  vessels,  and, 
dividing  and  subdividing,  accompany  them  in  the 
capsule  of  Glisson.  As  they  pass  inward  they 
become  smaller  and  smaller,  the  mucous  membrane 
loses  its  glands  and  becomes  simpler  in  structure — 
the  small  ducts  consisting  of  little  more  than  a 
simple  tube  lined  with  low  cylindrical  or  cuboidal 
cells.  Finally,  as  the  ducts  arrive  at  the  periphery 
of  the  lobules — interlobular  gall-ducts — they  are 
lined  with  flat,  polygonal  cells  ;  here  they  become 
continuous  with  the  intralobular  gall-passages  or 
gall-capillaries.     The   gall-capillaries   are   extremely 


THE  LIVER.  l6l 

narrow,  and  form  a  delicate  net-work  around  the 
individual  liver-cells;  being  arranged  in  such  a  way, 
however,  that  they  never  come  into  contact  with 
the  blood-capillaries,  always  being  separated  by  at 
least  a  part  of  the  diameter  of  a  liver-cell  from  the 
latter.  They  do  not  seem  to  possess  a  distinct  wall, 
but  are  rather,  simple  channels  grooved  in  the  walls 
of  contiguous  liver-cells. 

The  connective  tissue  of  the  human  liver  is  chiefly 
found  in  the  capsule  which  surrounds  the  organ,  and 
in  the  capsule  of  Glisson,  which  accompanies  the 
larger  vessels ;  but,  in  addition  to  this,  we  find  it  in 
very  small  quantity,  not  only  in  the  vicinity  of  the 
hepatic  vein,  but  also  between  the  cells  and  along 
the  capillaries  within  the  lobules ;  in  the  latter  situ- 
ation it  occurs  in  the  form  of  delicate  fibres  or 
membranes,  with  here  and  there  fusiform  or  stellate 
cells. 

The  lymphatic  vessels  of  the  liver  form  an  abun- 
dant net-work  in  the  capsule,  and  also  accompany 
the  larger  vascular  trunks  in  Glisson's  capsule  and 
between  the  lobules,  and  are  connected  with  minute 
intralobular  lymph-spaces.  Here  and  there  in  the 
interstitial  tissue  of  the  liver  are  found  small  irregu- 
lar nodules  of  lymphoid  tissue,  see  page  134. 

PRACTICAL    STUDY. 

Liver-cells. — A  small  fragment  of  fresh  liver  is  teased 
in  ^alt  solution  and  studied  in   the  same.     T^  nfecf  not  bo 


1 62  NORMAL  HISTOLOGY. 

preserved,  as  hardened  cells  will  be  seen  in  the  following 
preparations. 

Sections  of  Pig's  Liver. — A  small  piece  of  pig's  liver 
should  be  hardened  in  Mailer's  fluid  and  alcohol,  and 
very  thin  sections  made  near  the  surface,  both  parallel 
and  at  right  angles  to  it,  so  as  to  cut  the  lobules,  which 
are  quite  regularly  arranged  at  the  surface,  in  different 
directions.  The  sections  are  stained  double  and  mounted 
in  balsam.  The  lobules  being  here  surrounded  by  toler- 
ably distinct  layers  of  connective  tissue,  in  which  the  in- 
terlobular vessels  run,  the  lobular  structure  is  quite  evi- 
dent, and  the  pictures  obtained  by  sections  in  different 
directions  are  easy  of  interpretation. 

Sections  of  Human  Liver. — Sections  are  made  from  a 
bit  of  human  liver  hardened  as  above,  stained  double  and 
mounted  in  balsam.  Here  the  lobular  structure  is  very 
ill-defined,  because  of  the  small  amount  of  connective 
tissue  between  the  lobules,  and  for  the  recognition  of  the 
different  parts  of  the  latter  we  are  largely  dependent 
upon  the  determination  of  the  different  kinds  of  blood- 
vessels, since  these  always  bear  a  definite  relation  to  the 
lobules.  It  is  to  be  remembered  that  the  branches  of 
the  portal  vein  lie  only  in  the  periphery  of  the  lobules, 
that  they  are  usually  accompanied  by  other  vessels  be- 
sides capillaries,  and  are  in  most  cases  surrounded  by  a 
greater  or  less  amount  of  connective  tissue.  The  central 
vein,  on  the  other  hand,  is  usually  unassociated  with 
other  dissimilar  vessels,  except  capillaries,  and  sur- 
rounded only  by  a  scarcely  appreciable  amount  of  con- 
nective tissue. 

J 'ijcctcn  Liver. — The  general  arrangement  of  the  blood- 


THE  LIVER.  163 

vessels  is  best  studied  in  sections  from  a  human  or  rab- 
bit's liver,  which  has  been  injected  through  the  portal 
vein  with  the  blue  gelatin  mixture.  The  sections  are 
stained  with  eosin  and  mounted  in  balsam. 

Injected  Gall-capillaries. — By  a  careful  injection  of  the 
blue  gelatin  mixture  into  the  hepatic  duct,  the  gall-capil- 
laries may  be  partially  filled.  Sections  are  stained  with 
eosin  and  mounted  in  balsam. 

To  show  the  relation  between  the  blood-  and  gall-capil- 
laries, both  may  be  simultaneously  injected  with  gelatin 
of  different  colors,  one  forced  into  the  hepatic  duct,  the 
other  into  the  portal  vein. 


CHAPTER  XII. 

SUPRA-RENAL  CAPSULES— THYROID  GLAND. 

SUPRA-RENAL    CAPSULES. 

In  sections  of  the  supra-renal  capsules,  through 
their  thickest  part,  and  at  right  angles  to  the  long 
axis  of  the  organ,  we  see  with  the  naked  eye  two 
distinct  layers  ;  a  tolerably  firm,  yellowish,  striated 
cortical  layer  of  considerable  thickness — the  cortex, 
— and  a  narrower  central,  yellow  or  reddish  portion 
— the  medulla.  Between  the  two,  or  rather  at  the 
inner  side  of  the  cortical  portion,  a  more  or  less  dis- 
tinct brown  zone  is  seen. 

If  we  examine  thin  sections  microscopically,  we 
find  that  the  organ  is  enclosed  in  a  firm  connective- 
tissue  capsule,  in  which  are  numerous  elastic  fibres 
and  smooth  muscle-cells.  From  this  capsule,  deli- 
cate converging  connective-tissue  bands  or  septa, 
similar  in  structure  to  the  capsule,  pass  inward,  and 
being  joined  together  by  delicate  transverse  bands 
or  trabecular,  divide  the  cortex  into  a  multitude  of 
variously  shaped  chambers.  In  the  periphery,  just 
beneath  the  capsule,  the  chambers  are  small  and 
irregular  in  shape ;  then  comes  a  broader  zone, 
whose  chambers  are  long  and  narrow  ;  and  again,  at 

164 


SUPRA-RENAL  CAPSULES.  1 65 

the  inner  border  of  the  cortex,  they  are  small  and 
irregular.  In  the  medulla  the  supporting  frame- 
work has  the  form  of  a  delicate  reticulum  with  small 
irregular  meshes.  The  spaces  thus  formed  are  filled 
with  cells — -parenchyma  cells, — which  differ  in  char- 
acter in  the  different  parts  ;  in  the  cortex  they  are, 
for  the  most  part,  large,  polyhedral,  and  granular  ; 
less  abundant  are  smaller  cuboidal  or  cylindrical 
forms.  Those  in  the  elongated  chambers  are  usually 
crowded  with  fat-droplets,  and  those  lying  in  the 
inner  zone  usually  contain  an  abundance  of  brown 
pigment,  forming  the  above-mentioned  brown  zone. 
The  meshes  of  the  medulla  are  filled  with  large  globu- 
lar or  angular  or  branched,  finely  granular  cells, 
which,  in  marked  contrast  to  the  cortical  cells,  are 
stained  intensely  brown  by  solutions  of  chromic  acid 
or  its  salts. 

The  blood-vessels  are  very  abundant ;  many  of 
them  have  very  thin  walls  and  lie  in  close  contact 
with  the  cells  of  the  parenchyma.  The  organ  is 
abundantly  supplied  with  nerves  which,  passing 
along  the  converging  trabeculae,  form  a  dense  plexus 
in  the  medulla,  in  which,  as  well  as  in  the  capsule, 
considerable  numbers  of  ganglion-cells  are  found. 
Lymphatic  vessels  and  sinuses  are  also  abundant. 

PRACTICAL   STUDY. 

Sections. — A  human  supra-renal  capsule,  as  fresh  as 
possible — or,    if   this    cannot   be    obtained,    that    of   the 


1 66  NORMAL  HISTOLOGY. 

guinea-pig  or  ox, — is  cut  transversely  into  two  or  three 
pieces  and  hardened  in  bichromate  of  potassium  and 
alcohol.  Transverse  sections  are  stained  double  and 
mounted  in  glycerin  or  balsam. 

THE    THYROID    GLAND. 

The  thyroid  gland  is  composed  of  a  congeries  of 
larger  and  smaller  spheroidal  or  irregular-shaped 
alveoli,  enclosed  in  connective  tissue  and  grouped 
together  to  form  lobules.  The  alveoli  are  entirely 
separate  from  one  another,  and  they  have  no  excre- 
tory ducts.  Each  alveolus  has  a  delicate  membrana 
propria,  and  is  lined  with  a  single  layer  of  cylindri- 
cal or  cuboidal  cells.  The  connective  tissue  between 
the  alveoli  contains  numerous  blood-  and  lymphatic- 
vessels. 

The  entire  gland,  which  consists  of  two  lateral 
lobes,  united  at  their  lower  extremities  by  a  trans- 
verse commissure,  is  enclosed  in  a  dense  connective- 
tissue  envelope.  The  alveoli  are  filled  with  a  clear, 
homogeneous  albuminous  fluid,  which  in  adult  life 
is  frequently  transformed  into  or  replaced  by  a 
translucent  material,  called  colloid.  Owing  to  the 
pressure  which  the  accumulating  colloid  substance 
exerts  on  the  epithelium  lining  the  alveoli,  they  are 
often  very  much  flattened,  and  not  infrequently 
almost  entirely  disappear.  The  colloid  material 
seems  to  be  formed,  in  part  at  least,  by  a  transforma- 
tion of  the   contents  of  the   epithelial  cells,  and  its 


THE  THYROID  GLAND.  1 67 

formation   under  pathological  conditions  gives  rise 
to  one  of  the  forms  of  goitre. 

PRACTICAL   STUDY. 

The  thyroid  of  a  child  or  adult — better  the  former — is 
cut  into  small  pieces  and  hardened  in  alcohol.  Sections 
are  stained  double  and  mounted  in  glycerin  or  balsam. 


CHAPTER  XIII. 
THE  RESPIRATORY  APPARATUS. 

The  respiratory  apparatus  consists  of  a  multi- 
tude of  small  cavities  or  chambers,  on  whose  walls 
the  blood  is  brought  into  close  contact  with  the  air, 
and  of  a  system  of  branching  tubes  through  which 
the  air  is  conducted  to  and  from  them.  The  con- 
ducting tubes  are  the  larynx,  the  trachea,  and  the 
bronchi.  We  shall  limit  our  study  of  the  tubes  to 
the  two  last. 

The  walls  of  the  trachea  consist  of  several  layers : 
commencing  at  the  outside,  we  have  first,  a  layer 
of  firm  connective  tissue — the  fibrous  layer — in  which 
lie  embedded,  incomplete  cartilaginous  rings,  the 
space  between  whose  free  ends,  at  the  posterior  por- 
tion of  the  tube,  is  bridged  over  by  transverse  bands 
of  smooth  muscular  tissue,  which  bind  the  ends  to- 
gether. The  cartilaginous  rings  are  of  the  hyaline 
variety,  and  their  perichondrium  is  continuous  with 
the  looser  connective  tissue  of  the  layer  in  which 
they  lie.  This  layer  merges  into  the  next,  the  sub- 
mucosa,  which  consists  of  loose,  fibrillar  connective 
tissue  with  elastic  fibres,  the  latter  running  chiefly 
in  a  longitudinal  direction. 

168 


THE  RESPIRA  TOR  ] '  A  PPA  RA  TUS.  1 69 

Still  farther  inward,  and  not  distinctly  separated 
from  the  submucosa,  lies  the  mucosa,  composed  of 
fibrillar  connective  tissue  containing  an  abundance  of 
variously  shaped  cells,  and,  for  the  most  part,  longi- 
tudinally arranged  elastic  fibres.  In  some  parts  of 
the  mucosa  the  tissue  resembles  diffuse  lymphoid 
tissue.  The  connective  tissue  of  the  mucosa,  in 
some  parts  of  the  trachea,  does  not  form  an  uniform 
layer,  but  is  arranged  in  more  or  less  well-defined 
longitudinal  bundles,  giving  the  surface  a  wavy  or 
folded  appearance. 

Internally,  the  mucosa  is  bordered  by  a  thin, 
homogeneous  membrane,  the  basal  membrane,  upon 
which  the  epithelial  cells  lining  the  trachea  rest. 
The  epithelial  cells  are  usually  arranged  in  about 
three  layers.  Lying  upon  the  basal  membrane  are 
irregularly  spheroidal,  often  somewhat  elongated 
cells;  upon  these  lie  fusiform  or  pear-shaped  cells, 
while  the  surface  is  formed  by  a  layer  of  pyramidal 
or  cylindrical  ciliated  cells.  In  the  submucosa,  and 
sometimes  extending  outward  along  and  between 
the  cartilaginous  rings,  lie  racemose  mucous  glands, 
whose  excretory  ducts,  lined  with  cylindrical  epi- 
thelium, pass  obliquely  inward  and  terminate  on  the 
surface  in  expanded  orifices.  The  alveoli  of  the 
glands  are  lined  with  a  single  layer  of  slightly  granu- 
lar, polyhedral  cells.  When  the  glands  are  in  a  con- 
dition of  functional  activity,  however,  the  cells 
become  larger,  the  outlines   indistinct,  their  nuclei 


I70  NORMAL  HISTOLOGY. 

are  crowded  to  one  side,  and  their  contents  are 
apparently  transformed  into  a  homogeneous  mu- 
cous mass.  Scattered  here  and  there  among  the 
glands  and  between  the  cartilaginous  rings,  lie  larger 
and  smaller  clusters  of  fat-cells. 

The  blood-vessels,  passing  through  the  outer  lay- 
ers, furnish  an  abundant  capillary  net-work  to  the 
mucous  glands,  and  spread  out  in  a  rich  capillary 
plexus  beneath  the  basal  membrane. 

Although  the  larger  and  medium-sized  bronchi 
have  the  same  general  structure  as  the  trachea,  still 
we  find,  aside  from  a  decrease  in  the  thickness  of 
the  walls,  certain  noteworthy  structural  modifica- 
tions which  chiefly  concern  the  cartilaginous  and 
muscular  elements.  The  cartilage  occurs  in  the 
form  of  regular,  incomplete  rings,  only  in  the  upper 
portion  of  the  larger  bronchi.  As  the  tubes  become 
smaller  the  cartilage  occurs  in  the  form  of  scattered, 
irregular-shaped,  often  angular  plates,  which  become 
gradually  smaller  and  thinner.  Furthermore,  a  dis- 
tinct layer  of  smooth  muscular  tissue,  in  the  form  of 
transverse  rings,  connected  with  each  other  by  in- 
terlacing cells,  appears  between  the  mucosa  and  the 
submucosa.  We  find,  also,  that  the  longitudinal 
bundles  of  fibrillar  and  elastic  connective  tissue  in 
the  mucosa  are  more  strongly  developed  as  the 
bronchi  become  smaller,  so  as  to  throw  the  mucous 
membrane  into  pronounced  longitudinal  folds. 

Following  the   bronchi    now    down  toward  their 


THE  RESP1RA  TOR  Y  APPARA  TUS.  I  J  I 

finer  ramifications,  we  find  that  the  outer  connective- 
tissue  layer  becomes  thinner,  the  cartilaginous  plates 
become  smaller  and  more  infrequent,  and  finally 
altogether  disappear;  the  mucous  glands,  too,  after 
becoming  smaller  and  simpler  in  structure,  disap- 
pear with  the  cartilages.  The  muscular  rings  as- 
sume, gradually,  the  form  of  an  uniform  thin  layer 
of  transversely  arranged  muscle-cells,  intermingled 
with  elastic  fibres,  and  are  finally  represented  only 
by  a  few  scattered  transverse  cells.  The  mucosa  in 
the  smaller  tubes  becomes  gradually  thinner,  and 
finally  merges  into  the  fibrous  layer,  with  the  inter- 
vention only  of  a  few  scattered  muscle-cells ;  the 
lower  layers  of  epithelium  gradually  disappear,  leav- 
a  single  row  of  ciliated  cells  upon  the  basal  mem- 
brane. Finally,  we  have,  in  the  smallest  tubes,  very 
thin  connective-tissue  walls,  containing  a  few  mus- 
cle-cells and  elastic  fibres,  and  lined  with  cuboidal, 
ciliated,  and  at  last  with  flattened,  non-ciliated  cells. 
We  are  thus  led  to  the  respiratory  cavities  of  the 
lungs — the  air-chambers.  If  we  look  at  the  surface 
of  a  lung,  we  see  that  it  is  more  or  less  distinctly 
divided,  by  narrow  branching  lines,  into  irregular 
polygonal  spaces,  each  one  of  which  corresponds  to 
a  pulmonary  lobule.  These  lobules  have,  on  the  sur- 
face of  the  lung,  where  they  are  more  uniform  in 
shape  than  within,  a  pyramidal  form,  and  are  sepa- 
rated by  narrow  connective-tissue  septa,  and  each 
lobule  is,  in  fact,  a  group  of  air-vesicles,  which  are 


\J2  NORMAL  HISTOLOGY. 

in  communication  with  a  single  small  bronchus  ~nd 
its  terminal  twigs.  As  the  small  bronchus  enters 
the  lobule,  it  divides  into  several  terminal  twigs  ; 
these  open  at  the  peripheral  extremity  into  irregular 
tubular  passages,  called  alveolar  passages ;  on  the 
sides  of  these  alveolar  passages  still  other  irregular, 
sometimes  branched  elongated  cavities,  called  in- 
fundibula, open  out,  and  their  walls,  as  well  as  the 
walls  of  the  alveolar  passages,  are  beset  with  tiny, 
irregular-shaped  vesicles,  called  air  vesicles  or  alveoli, 
which  communicate  with  the  cavities  of  the  infun- 
dibula  and  alveolar  passages  by  wide,  irregular  open- 
ings. All  of  these  cavities  are  closely  crowded  to- 
gether, and  their  walls  intimately  joined.  It  is  in  the 
walls  of  the  air-vesicles  that  that  interchange  of 
material  between  air  and  blood  occurs,  which  is  the 
essential  factor  in  respiration. 

We  have  now  to  consider  the  structure  of  the 
walls  of  the  alveolar  passages,  infundibula,  and  air- 
vesicles.  The  gradual  thinning  which  we  have  ob- 
served in  the  walls  of  the  bronchi  as  they  approach 
their  termination,  is  still  more  marked  as  we  pass 
over  into  the  alveolar  passages  and  infundibula. 
Here,  the  walls  consist  of  little  else  than  a  thin, 
delicately  striated,  membranous  basement  substance, 
in  which  numerous  elastic  fibres  rarrfify,  and  a  few 
connective-tissue  and  smooth  muscle-cells  are  em- 
bedded, the  whole  being  lined  with  flattened  epithe- 
lium.    At  the  opening  of  the  infundibula  and  air- 


THE  RESPIRA  TOR  Y  A  PPA  RA  TUS.  I  /  3 

vesicles  the  clastic  fibres  are  grouped  to  form  pro- 
jecting rings,  which  bound  the  opening.  From  these 
rings  of  elastic  fibres  which  surround  the  openings 
into  the  alveoli,  other  elastic  fibres  are  given  off, 
which,  dividing  and  subdividing,  stretch  over  the 
walls  of  the  air-vessels  in  the  form  of  a  wide-meshed 
net,  the  spaces  between  the  fibres  being  occupied 
by  an  extremely  thin,  structureless  membrane,  in 
which  lies  an  occasional  oval  nucleus. 

The  alveoli,  in  the  adult,  are  lined  with  a  single 
layer  of  flattened,  polygonal,  epithelial  cells.  These 
are  of  two  kinds:  first,  small  granular,  nucleated 
cells  ;  and,  second,  cells  which  are  larger,  more  irreg- 
ular in  form,  very  thin  and  transparent,  and  usually 
without  nuclei.  The  relative  proportion  of  these 
two  kinds  of  cells  is  variable,  and  their  outlines, 
especially  those  of  the  larger  cells,  it  is  difficult  to 
see  distinctly  without  resorting  to  silver-staining. 

In  the  fcetus  the  air-vesicles  are  lined  at  first  with 
a  distinct  layer  of  cylindrical  or  cuboidal  epithelial 
cells,  which  become  gradually  flattened,  and  when 
respiration  is  established  assume  the  form  of  a  layer 
of  very  thin  polygonal  cells  ;  these  change  their 
character  as  the  animal  matures,  until  in  adult  life 
we  have  the  forms  above  described. 

Just  beneath  the  epithelial  cells,  and  separated 
by  them  alone  from  the  air  within  the  vesicles,  lies 
the  rich  blood-capillary  net-work,  which,  in  bulk  as 
in  importance,  is  the  most  essential  element  in  the 


174  NORMAL  HISTOLOGY. 

walls  of  the  vesicles.  The  lungs  are  supplied  with 
blood  through  the  pulmonary  and  the  bronchial 
arteries.  The  blood  from  the  latter  is  chiefly  dis- 
tributed to  the  walls  of  the  bronchi  and  larger 
blood-vessels,  and  to  the  connective  tissue  of  the 
lungs.  A  large  part  of  the  blood  from  the  bron- 
chial arteries,  after  passing  through  various  sets  of 
capillaries,  returns  through  the  bronchial  veins ;  but 
a  certain  portion  of  it  finds  its  way  into  the  pulmon- 
ary veins ;  indeed,  the  two  sets  of  vessels  seem  to 
be  in  communication  in  various  parts  of  the  lungs. 

The  pulmonary  artery,  following  the  course  of 
the  bronchi,  divides  and  subdivides,  until  on  reach- 
ing the  lobules  the  small  trunks  break  up  into 
smaller  branchlets,  which  pass  along  the  alveolar 
passages  and  in  the  interlobular  connective  tissue  to 
the  air-vesicles,  where  they  break  up  into  the  rich 
capillary  net-work  which  is  spread  over  their  walls. 
The  capillaries  wind  over  the  free  edges  of  the  al- 
veolar walls,  the  vessels  often  projecting  somewhat 
into  their  cavities.  The  net-work  on  the  alveolar 
i  walls  is  quite  dense,  and  the  vessels  are  very  broad, 
leaving  only  small  oval  or  rounded  spaces  between 
them.  A  single  net-work  frequently  supplies  the 
walls  of  adjacent  vesicles,  which  in  such  cases  are 
merged  into  one.  The  blood  passes  from  the  vesi- 
cles into  the  pulmonary  veins  in  the  interlobular 
connective  tissue,  and  then  into  larger  trunks  which, 
passing  inward,  follow  the  course  of  the  other  large 
vessels. 


THE  RESP1RA  TOR  V  ATTAR  A  TUS.  1 75 

The  surface  of  the  lungs  is  invested  with  a  thin 
layer  of  connective  tissue, — the  pulmonary  pleura, — 
which  contains  numerous  blood-  and  lymph-vessels, 
and  is  covered  with  endothelium.  Here  and  there, 
beneath  the  pleura,  as  well  as  elsewhere  in  the 
lungs,  between  the  lobules  and  around  the  alveolar 
passages  and  small  bronchi,  are  small  irregular 
nodules  of  lymphoid  tissue. 

The  epithelial  cells  of  the  alveoli  often  contain 
brown  or  black  pigment,  and  pigment  deposits  are 
of  the  most  frequent  occurrence,  in  the  adult  lung, 
in  the  interlobular  connective  tissue,  and  in  the  con- 
nective tissue  and  lymph  nodes  at  the  base  of  the 
lungs,  as  well  as  in  the  above-mentioned  lymphoid 
nodules.  The  greater  part  of  this  pigment  is  prob- 
ably taken  up  from  the  respired  air,  and  transported 
through  the  cells  or  lymph-channels  to  the  parts  in 
which  it  is  found. 

PRACTICAL   STUDY. 

Trachea. — A  portion  of  the  human  trachea  is  placed 
for  ten  days  in  -J-per-cent.  solution  of  chromic  acid,  and 
the  hardening  completed  with  alcohol.  A  small  bit  is 
embedded  in  wax  or  hardened  liver,  care  being  taken  not 
to  rub  off  the  ciliated  cells  in  the  manipulation,  and  the 
longitudinal  and  transverse  sections  are  stained  double. 
and  mounted  in  balsam. 

Uninfected  Lung. — The  lungs  from  the  human  subject, 
or  any  small  animal,  such  as  the  dog,  rabbit,  or  cat,  hav- 
ing  been   carefully  removed,  a  canula    is  tied   into  the 


176  NORMAL  HISTOLOGY. 

trachea  (or  one  of  the  large  bronchi,  if  one  lung  only  is 
to  be  prepared),  and  the  organ  is  distended  by  pouring  a 
one-sixth-per-cent.  solution  of  chromic  acid  into  the 
canula,  which,  for  this  purpose,  may  be  connected  by  a 
short  rubber  tube  with  a  funnel.  When  the  lung  is  filled 
and  its  surface  tense,  the  trachea  or  bronchus  is  tied,  and 
the  entire  organ  immersed  in  the  same  fluid  with  which 
its  alveoli  are  filled.  After  a  couple  of  days  the  organ 
may  be  cut  in  pieces  and  put  into  a  stronger  solution  of 
the  chromic  acid,  one-fifth  to  one-fourth  per  cent.  In 
three  or  four  days  they  are  washed  and  transferred  at 
first  to  dilute,  and  then  to  strong,  alcohol.  Sections  may 
be  stained  double  and  mounted  in  glycerin. 

To  demonstrate  the  elastic  fibres  in  the  walls  of  the  air- 
vesicles,  thin  sections  of  an  unstained  human  lung  are 
mounted  in  glycerin  to  which  strong  acetic  acid  has  been 
added  in  the  proportion  of  1-100. 

Epithelium  of  the  Air-vesicles. — The  lung  of  a  freshly- 
killed  animal  (a  young  cat  is  best)  is  filled  with  a  solution 
of  silver  nitrate  1-500,  in  the  manner  just  described,  and 
after  remaining  for  half  an  hour,  the  tube  between  the 
funnel  and  the  lungs  should  be  lengthened  to  about  six 
or  eight  inches,  so  as  to  increase  the  pressure,  and  the 
funnel  filled  with  a  mixture  of  equal  parts  of  alcohol  and 
water.  Under  this  increased  pressure,  the  silver  solution 
will  be  partially  driven  out  through  the  pleura  and  re- 
placed by  the  dilute  alcohol.  When  this  replacement  is 
partially  accomplished,  the  lung  is  placed  entire  in  alcohol 
of  the  same  strength  and  exposed  to  the  light.  After  a 
few  hours  it  may  be  cut  into  pieces  and  preserved  in 
strong    alcohol.      Sections   are   made    from    the     surfaces 


THE  RESPIRA  TOR  Y  APPARA  TUS.  1 77 

which    have   become    brown,   and    mounted    in    glycerin 
tinged  with  eosin. 

Injected  Lung. — From  bits  of  human  lung  whose  blood- 
vessels have  been  injected  with  the  blue  gelatin  mixture 
through  the  pulmonary  artery,  sections,  which  need  not 
be  very  thin,  are  made,  stained  with  eosin  and  mounted 
in  balsam. 


CHAPTER  XIV. 
THE  KIDNEY. 

If  a  longitudinal  section  be  made  through  the 
middle  of  a  rabbit's  kidney,  the  cut  surface  will  pre- 
sent a  well-marked  separation  into  an  outer  cortical 
and  an  inner  medullary  substance;  between  these 
two  are  seen  sections  of  large  blood-vessels.  The 
medullary  portion,  called  medullary  pyramid  ox  me- 
dulla, and  occupying  the  central  part  of  the  organ, 
terminates  in  the  pelvis  in  a  single  short,  rounded 
prolongation,  called  the  papilla,  at  the  end  of  which, 
with  a  low  magnifying  power,  several  tiny  openings 
may  be  seen.  The  cut  surface  of  the  papilla  pre- 
sents an  uniform  grayish  appearance,  while  the  seg- 
ment of  the  medulla  adjacent  to  the  cortex  presents 
distinct  bands  or  striae,  radiating  from  the  papilla 
and  extending,  in  the  form  of  narrow,  isolated 
tapering  rays,  into  the  cortex,  almost  to  the  surface 
of  the  organ  ;  these  cortical  rays  are  called  medtillary 
rays. 

The  cortex,  surrounding  the  medulla  like  a  thick 
shell,  and  covered  by  a  firm,"  dense  layer  of  connec- 
tive tissue — the  capsule — consists,  besides  the  medul- 

i73 


THE  KIDXEY.  1 79 

lary  rays,  of  a  grayish  substance  lying  in  the  form 
of  elongated  truncated  pyramids  between  the  rays, 
d  in  a  thin  irregular  layer  of  the  same  appearance 
rectly  beneath   the   capsuk.      The  grayish,  pyra- 
midal   portions    of    the    cortex-  are    called    cortical 
.  and   the  entire   substance  of   the  cortex, 
exclusive  of  the  medullary  rays,  is  sometimes  called 
the  labyrinth.     If  the   blood-vessels   of   the  kidney 
have  been   injected  with  some  colored  substance,  or 
it  the  vessels  are  well  filled  with  blood,  tiny  vessels 
can  be  seen  passing  off  from  the  large  trunks  which 
lie  beween  the   cortex  and  medulla  ;  and  extending 
radially  toward  the  surface  of  the  organ  through  the 
centre  of  the  cortical  pyramids,  and  at  each  side  of 
these  vessels,  may  be  seen  a  row  of  minute  globular 
structures,    which    are    the    Malpighian    bodies    or 
glomeruli. 

The  human  kidney  differs  from  that  which  we 
have  just  described,  in  that  it  consists  of  a  number 
of  just  such  structures  crowded  together  to  form  a 
single  organ.  In  embryonic  life  its  composite  nature 
i  s  evident,  because  each  renculus — as  each  portion 
corresponding  to  the  rabbit's  kidney  is  called— is 
separated  from  its  neighbors  by  a  certain  amount  of 
connective  tissue,  giving  the  surface  of  the  organ  a 
lobulated  appearance.  As  the  individual  matures, 
therenculi  usually  become  merged  into  one  another, 
so  that  we  no  longer  see  on  the  surface  any  trace  of 
its  composite  character.     This  is  betrayed,  however, 


ISO  NORMAL  HISTOLOGY. 

by  the  fact  that  the  medullar}'  pyramids  of  the 
primitive  renculi  persist  as  separate  structures,  and 
we  thus  have  in  the  adult  human  kidney  just  as 
many  medullary  pyramids  and  papillae  as  there  were 
original  renculi.  We  find  also,  which  is  a  striking 
feature  in  the  adult  human  kidney,  that  the  cortical 
substance  is  not  confined  to  the  cortex,  but  extends 
into  the  pelvis  between  the  papillae,  often  as  far  as, 
and  sometimes  farther  than  the  papillae  themselves. 
Not  very  infrequently  the  divisions  between  the 
primitive  renculi  are  not  entirely  obliterated  in  the 
process  of  development,  and  the  surface  of  the  kid- 
ney, even  in  the  adult,  is  distinctly  lobulated. 

Having  thus  acquainted  ourselves  with  the  general 
structure  of  the  kidney,  we  have  now  to  study  the 
elements  of  which  it  is  composed,  and  the  way  in 
which  they  are  grouped  to  form  the  different  parts 
above  described.  The  kidney  is  a  tubular  gland; 
the  innumerable  tubes  of  which  it  is  mainly  com- 
posed are  lined  with  epithelial  cells,  and  run  a  very 
tortuous  course  from  their  origin  in  the  cortex  to 
their  termination  in  the  tiny  openings,  above  men- 
tioned, at  the  apex  of  the  papillae.  They  constitute, 
with  the  cells  lining  the  walls  of  the  glomeruli,  in 
which  they  originate,  the  parenchyma  of  the  kidney. 
In  addition  to  the  parenchyma  we  have,  then,  to 
study  the  connective  tissue  or  interstitial  tissue  of 
the  organ  and  the  blood-vessels. 

The  tubes  of  the  kidney,  called  in  general  urin- 


THE  KIDNEY.  l8l 

ifcrous  titbit les,  consist  of  an  apparently  homogeneous 
membrana  propria  lined  throughout  with  a  single 
layer  of  epithelial  cells,  which  differ  greatly  in  char- 
acter and  form  in  different  parts  of  the  tubules;  the 
tubules  have  received  special  names  in  different 
parts  of  their  course. 

Each  tubule  commences  within  a  cortical  pyramid, 
in  a  dilatation  called  the  glomerulus,  with  whose 
capsule,  presently  to  be  described,  its  membrana 
propria  is  continuous  ;  it  is  narrow  as  it  leaves  the 
glomerulus,  but  broadens  out  at  once  into  a  wide, 
convoluted  canal — convoluted  tubule — which  winds 
about  in  the  pyramid,  finally  approaching  a  medul- 
lary ray;  this  it  enters,  and,  suddenly  becoming  very 
narrow,  descends  more  or  less  deeply  into  the 
medulla;  here,  widening  somewhat,  it  turns  sharply 
on  itself  and  ascends  into  a  medullary  ray  again. 
This  portion  of  the  tube,  from  the  point  at  which  it 
becomes  narrow  and  begins  to  descend  in  the 
medullary  ray,  is  called  Henles  loop,  and  the  arms  of 
the  loop  are  called,  following  the  course  which  the 
description  has  taken,  the  descending  ox  narroiv,  and 
the  ascending  or  broad  arms  of  Henle's  loop.  The 
ascending  arm  of  Henle's  loop,  on  arriving  in  the 
cortex,  widens  and  enters  a  cortical  pyramid,  form- 
ing what  is  known  as  the  intercalated  tubule ;  this 
resembles  the  convoluted  tubules,  among  which  it 
winds  in  and  out,  and  then  passes  over,  entering  a 
medullary  «ray,   into    a  straight    urinifcrous   tubule. 


1 82  NORMAL  HISTOLOGY. 

This  is  again  narrower  than  the  convoluted  tubules, 
and  passes  directly  downward  through  the  medulla, 
joining  other  similar  tubules  dichotomously  and 
becoming  larger  as  it  does  so,  until  at  length  it 
opens  at  the  apex  of  a  papilla.  The  uriniferous 
tubules  run  an  entirely  independent  course  until,  as 
straight  tubules,  they  join  one  another  by  twos,  to 
form  the  outlet  ducts. 

We  have  now  to  consider  the  epithelium  which 
lines  the  tubules.  Commencing  at  the  straight 
tubules  in  the  papillae,  we  find  that  in  their  lower 
portion  they  are  lined  by  cylindrical  cells  with  large 
nuclei  and  transparent  bodies ;  further  up  the  cells 
become  more  nearly  cuboidal,  and  are  often  flattened 
in  the  medullary  rays.  The  epithelium  of  the  con- 
voluted and  intercalated  tubules  is  similar  in  charac- 
ter, and  consists  of  large  granular  striated  cells, 
whose  outlines  are  not  well  defined,  and  which 
nearly  fill  the  lumen  of  the  tube.  The  ascending 
arm  of  Henle's  loop  is  lined  with  pyramidal  or  low 
cylindrical,  granular  cells ;  while  the  very  narrow 
descending  arm  is  lined  with  flat,  transparent  cells, 
whose  nuclei  usually  project  into  the  lumen  of  the 
tube. 

The  glomeruli  consist,  in  the  first  place,  of  a  mem- 
branous, apparently  structureless  capsule,  similar  to 
and  continuous  with  the  membrana  propria  of  the 
tubules.  The  epithelium  of  the  convoluted  tubules, 
as  the  latter  join  the  glomeruli,  becomes  flattened 


THE  KIDNEY.  I  83 

and  continuous  with  a  layer  of  very  thin  transparent 
cells,  which  completely  line  the  capsule.  At  one 
side  of  the  glomerulus,  usually  opposite  to  the  at- 
tachment of  the  tubule,  a  small  artery — the  vas 
affcrens — pierces  the  capsule  and  immediately  di- 
vides into  a  number  of  capillary  loops,  which  wind 
about  one  another,  forming  a  complicated  vascular 
tuft  ;  the  blood  from  this  tuft  is  collected  into  a 
vein — vas  efferens — which  is,  as  a  rule,  somewhat 
smaller  than  the  afferent  artery,  and  leaves  the 
glomerulus  near  the  point  where  the  latter  enters. 
The  capillary  tuft  within  the  glomerulus  is  cov- 
ered also  with  a  layer  of  flat  cells  like  those  lining 
the  capsule. 

Let  us  now  review  the  positions  of  the  different 
parts  of  the  tubules  in  the  kidney.  In  the  papillae 
are  the  termini  of  the  straight  tubules ;  in  the 
medulla  lie  the  straight  tubules  and  portions  of 
Henle's  loops  ;  in  the  medullary  rays  are  the  upper 
portions  of  the  straight  tubules,  and  of  both  arms 
of  Henle's  loops  ;  in  the  labyrinth,  including  the 
cortical  pyramids,  are  the  convoluted  tubules,  inter- 
calated tubes,  and  the  glomeruli. 

The  distribution  of  the  blood  in  the  kidney  yet 
remains  to  be  considered.  The  larger  branches  of 
the  renal  arteries,  accompanied  by  the  veins,  enter 
the  organ  at  the  bases  of  the  medullary  pyramids, 
and  divide  into  large  arching  trunks  which  pass,  in 
various  directions,  with  their  convexity  toward  the 


1 84  NORMAL  HISTOLOGY. 

cortex,  along  the  irregular  boundary  line  between 
the  cortex  and  medulla.  From  the  convex  side  of 
the  arterial  trunks  spring  numerous  small  branches, 
each  of  which  enters  at  once  the  apex  of  a  cortical 
pyramid,  and  proceeds  directly  toward  the  surface 
of  the  organ ;  these  arteries  are  called  interlobular 
arteries.1  From  these  interlobular  arteries  lateral 
twigs  are  given  off  at  frequent  intervals,  which,  after 
passing  a  short  distance,  enter  the  glomeruli  as  the 
arteriae  afferentes.  On  leaving  the  glomerulus,  the 
efferent  vein — which  still  carries  arterial  blood — 
breaks  up  into  a  capillary  net-work,  which  lies 
among  the  adjacent  convoluted  tubules  and  in  the 
neighboring  medullary  rays,  the  meshes  in  the  for- 
mer region  being  rounded,  in  the  latter,  elongated, 
corresponding  to  the  character  of  the  tubules  among 
which  they  lie.  From  these  the  blood  is  collected 
into  small  veins,  which  in  turn  pour  it  into  interlob- 
ular veins,  and  these,  following  the  course  of  the 
interlobular  arteries,  finally  pour  it  into  the  large 
arched  trunks  between  the  cortex  and  medulla.  In 
the  superficial  portions  of  the  cortex  there  are  no 
glomeruli,  and  here   small  venous  trunks   centre  in 

1  If  we  consider  a  circumscribed  portion  of  the  cortex  of  the  kid- 
ney, having  for  its  centre  a  medullary  ray,  and  extending  on  every 
side  as  far  as  to  the  nearest  interlobular  vessels,  we  see  that  in  this 
limited  area  we  have  all  of  the  essential  structural  elements  of  the 
coriex — a  medullary  ray  surrounded  by  convoluted  tubules  and  glomer- 
uli. Such  groups  of  elements,  although  they  have  by  no  means  a 
separate  existence,  and  can  be  but  indefinitely  bounded,  have  been 
called  iobuli ;  and,  hence,  the  vessels  passing  between  them  are  termed 
interlobular  vessels. 


THE  KIDNEY.  1 85 

the  commencement  of  an  interlobular  vein,  forming 
the  well-known  stcllulce  Vcrheyenii. 

The  medulla  receives  its  blood  in  part  from  the 
vasa  efferentes  of  the  glomeruli,  which  lie  near  the 
boundary,  between  cortex  and  medulla ;  in  greater 
quantity  from  the  large  arterial  arches.  Vessels 
from  the  latter  sources  descend  in  spreading  tufts, 
called  vasa  recta,  between  the  tubules,  and  break  up 
into  a  long-meshed  capillary  net,  the  blood  from 
which  is  collected  in  part  into  a  round-meshed 
venous  net-work  in  the  papillae,  and  returned  in 
straight  veins  along  the  tubules  to  the  venous 
arches,  and  in  part  passes  directly  back  to  the 
arches  without  going  down  to  the  papillae.  The 
capsule  of  the  kidney  receives  its  blood  in  part 
from  the  terminal  twiglets  of  the  interlobular  ar- 
teries, in  part  from  terminal  branches  of  the  phrenic, 
lumbar,  and  supra-renal  arteries,  and  it  passes  into 
the  stellate  veins. 

The  connective  tissue  of  the  kidney,  aside  from 
that  which  forms  the  capsule,  lines  the  pelvis,  and 
is  distributed  along  the  walls  of  the  larger  blood- 
vessels and  around  the  glomeruli,  is  very  small  in 
amount.  It  may,  however,  be  demonstrated  here 
and  there,  and  is  most  abundant  in  the  vicinity  of 
the  papilla. 

PRACTICAL    STUDY. 

Rabbit's  Kidney — General  View.—K  rabbit's  kidney  is 
cut  transversely  across  the  middle  and   hardened  in  two- 


1 86  NORMAL  HISTOLOGY. 

per-cent.  solution  of  bichromate  of  potassium  and  alco- 
cohol.  Thin  sections  are  made  across  the  entire  organ, 
including  cortex,  medulla,  and  papilla,  stained  double 
and  mounted  in  balsam. 

Isolated  Tubules. — A  small  fragment  of  fresh  kidney, 
including  both  cortical  and  medullary  portions,  is  placed 
in  a  mixture  of  equal  parts  of  alcohol  and  strong  hydro- 
chloric acid  ;  the  acid  partially  dissolves  the  intertubular 
:issue,  so  that  after  a  time  the  tubules  can  easily  be  pulled 
apart.  This  is  usually  effected  in  about  twelve  hours  ; 
jut  the  specimen  should  be  examined  from  time  to  time 
after  it  has  been  in  the  acid  for  eight  hours,  and  removed 
as*soon  as  the  object  is  accomplished.  It  is  allowed  to 
soak  for  a  few  hours  in  water  to  remove  the  acid,  and 
then  small  bits  are  torn  off,  including  cortical  and  medul- 
lary substance,  and  the  tubules  carefully  separated  on  a 
slide.  They  are  extremely  brittle,  and  great  care  must 
be  used  in  their  isolation  ;  this  may  be  done  by  needles, 
or  better  by  allowing  small  drops  of  water  to  fall  upon 
the  specimen  on  a  slide,  from  a  pipette.  When  the  dis- 
sociation is  partially  effected,  the  excess  of  water  should 
be  removed  with  filter  paper,  and  a  mixture  of  equal  parts 
of  saturated  solution  of  picric  acid  and  glycerin  added, 
in  which  it  is  mounted  and  preserved. 

Sections  of  Uninjected  Human  Kidney. — A  perfectly 
fresh  human  kidney  is  cut  into  small  pieces  and  hardened 
in  strong  alcohol.  Sections  are  made  in  a  plane  vertical 
to  the  surface  of  the  organ,  including  both  cortical  and 
medullary  portions,  and  also  parallel  to  the  surface, 
through  the  cortex.  They  are  stained  double  and 
mounted  in  glycerin. 


THE  KIDNEY.  1 87 

Sections  of  Injected  Kidney. — The  kidney  is  injected, 
through  both  the  renal  artery  and  vein,  with  the  blue 
gelatin  mixture,  and  sections  in  different  directions  are 
stained  with  eosin  and  mounted  in  balsam. 


CHAPTER  XV. 
THE    GENERATIVE    ORGANS. 

MALE    GENERATIVE    ORGANS. 

WE  shall  confine  our  study  of  these  organs  to  the 
testicle,  prostate  gland,  and  parts  of  the  penis. 

THE    TESTICLE SPERMATOZOA. 

This  is  a  tubular  gland,  whose  chief  specific  secre- 
tion is  the  spermatozoa  ;  it  is  enclosed  by  a  firm, 
dense  connective-tissue  capsule,  called  the  albu- 
ginea,  which  sends  inward  several  incomplete  septa, 
which  divide  the  organ  into  a  number  of  communi- 
cating cavities,  and  unite  at  its  posterior  superior 
part  in  a  dense,  wedge-shaped  mass  of  connective 
tissue,  called  the  corpus  Highmori.  The  more  or 
less  conical  cavities  between  the  septa  contain  wind- 
ing, anastomosing,  and  looped  tubules,  called  semi- 
niferous tubules,  which  constitute  the  secreting  por- 
tion of  the  organ.  As  the  tubules  approach  the 
corpus  Highmori,  they  become  narrower  and 
straighter — tubuli  recti — and  are  lined  with  cylindri- 
cal epithelium,  and,  on  entering  that  body,  form  a 
net-like  series  of  intercommunicating  channels  lined 

183 


THE  GENERATIVE  ORGANS.  1S9 

with  flattened  cells,  called  the  rete  testis.  The  chan- 
nels of  the  rete  testis  are  continuous  with  several 
tubules,  which,  passing  upward  and  backward,  be- 
come very  much  convoluted,  and  form  a  number  of 
conical  masses — coni  z\iscu  ksi—\x\\\Q\\  largely  consti- 
tute the  head  of  the  epididymis.  The  tubules  of 
the  coni  vasculosi  gradually  unite  as  they  descend 
to  form  a  single  canal,  which,  with  numerous  wind- 
ings and  contortions,  constitutes  the  body  and  tail 
of  the  epididymis,  and  finally  becomes  continuous 
\\  ith  a  straight,  thick-walled  ascending  tube — the 
vas  deferens. 

The  seminiferous  tubules,  which  chiefly  concern 
us  here,  lie  embedded  in  loose,  delicate,  lamellated 
connective  tissue,  which  is  abundantly  supplied  with 
blood-vessels,  and  contains  man}-  cells  :  among  the 
ordinary  connective-tissue  cells  of  various  forms, 
large  granular,  often  pigmented  cells  are  not  infre- 
quently seen  lying  singly  or  in  groups,  and  some- 
times in  rows  along  the  blood-vessels  :  their  nature 
and  significance  are  still  doubtful.  The  tubules 
have  a  distinct  membrana  propria,  and  are  lined 
with  several  layers  of  cells  piled  irregularly  over 
one  another,  which  differ  in  form  under  different 
circumstances,  and  sometimes  in  different  parts  of 
the  same  gland. 

In  a  tubule  which  is  not  producing  spermatozoa, 
the  outer  row  of  cells — those  lying  upon  the  mem- 
brana propria — are  large,  granular,  well-defined,  nu- 


l9°  NORMAL  HISTOLOGY. 

cleated  cells ;  and  upon  these  lie  two  or  three  irreg- 
ular layers  of  smaller  nucleated  cells  with  ill-defined 
cell-bodies.  Between  the  cells  of  the  inner  layers 
a  peculiar,  irregular-branching  net-work  is  seen, 
which,  by  some  recent  observers,  is  believed  to  be 
formed,  for  the  most  part,  by  branches  of  the  outer 
row  of  cells  ;  and  it  is  supposed  that  it  serves  as  a 
loose  framework  in  which  the  inner  cells  are  sup- 
ported. Others  believe  it  to  be  only  intercellular 
cement-substance.  The  lumen  of  these  tubules 
may  be  filled  with  granular  material  or  may  con- 
tain spermatozoa. 

In  tubules  which  are  in  full  functional  activity, 
the  cavity  is  usually  filled  with  more  or  less  granular 
material  and  mature  and  immature  spermatozoa,  and 
on  every  side  a  multitude  of  clusters  or  bundles  of 
developing  spermatozoa  are  seen,  with  the  heads 
embedded  among  the  cells  of  the  inner  layers,  which 
are  similar  to  the  lining  cells  above  described,  and 
the  tails  stretching  brush-like  into  the  cavity  of  the 
tube. 

According  to  some  observers,  the  spermatozoa  are 
formed  from  certain  of  the  cells  of  the  inner  layers, 
which  are  called  spermatoblasts.  The  process  of  de- 
velopment, according  to  this  view,  commences  in  the 
muclei  of  the  spermatoblasts,  which  become  con- 
verted into  the  head,  while  the  tail  is  an  outgrowth 
from  the  nucleus,  or  is  produced  by  a  transformation 
of  a  portion  of  the  cell-protoplasm.     Others  believe 


THE  GENERATIVE  ORGANS.  I9I 

that  the  spermatozoa  are  formed  by  the  growth  in- 
ward, from  the  large  outer  cells,  or  from  cells  lying 
among  these,  of  a  long-stemmed  bud-like  process, 
whose  dilated  end  divides  into  a  number  of  longitud- 
inal segments,  each  of  which  finally  becomes  a  sper- 
matozoon. That  the  latter  is  the  mode  of  development 
in  certain  animals,  e.  g.  the  rat,  would  seem  to  be 
unquestionable. 

The  mature  spermatozoa  differ  in  form  in  different 
animals;  in  man  they  consist  of  a  flattened,  pear- 
shaped  portion,  called  the  head,  the  small  end  of 
which  is  directed  forward  ;  and  a  delicate,  tapering, 
almost  filiform  portion,  called  the  tail;  while  be- 
tween the  head  and  tail  is  a  short,  narrow  segment, 
called  the  middle  piece.  When  living,  and  under 
favorable  conditions,  the  spermatozoa  are  capable 
of  performing  rapid  movements,  the  whole  organism 
being  driven  hither  and  thither  by  wavy  vibrations 
of  the  tail. 

PRACTICAL    STUDY. 

Spermatozoa. — These  may  be  obtained  from  the  sem- 
inal vessels  of  man,  or  from  the  sediment  of  urine  in 
which  they  occur  either  normally,  or  under  pathological 
conditions.  They  are  well  preserved  by  a  mixture  of 
equal  parts  of  saturated  sol.  of  picric  acid,  glycerin,  and 
water,  in  which  they  may  be  mounted. 

Spermatozoa  for  comparative  study  may  be  readily  ob- 
tained by  making  an  incision  into  the  head  of  the  epi- 
didymis of  a  dog,  rabbit,  or  guinea-pig,  and  receiving  the 
milkv  fluid  which  exudes  in  the  above  nricric  acid  fluid. 


192  NORMAL  HISTOLOGY. 

The  movements  of  the  living  spermatozoa  may  be 
studied  by  mixing  some  of  the  milky  fluid  from  the  epi- 
didymis of  a  freshly  killed  animal,  on  a  warm  slide,  with 
J-per-cent.  salt  solution,  and  protecting  from  pressure 
by  a  hair. 

The  temperature  of  the  slide  should  be  kept  at  about 
the  same  elevation  as  that  of  the  animal  from  which  they 
are  taken.  The  movement  may  be  stopped  by  the  ad- 
dition of  water. 

Sections  of  Testicle. — Small  pieces  of  the  human  tes- 
ticle, obtained  as  soon  as  possible  after  death,  or,  if  this 
cannot  be  obtained,  of  the  testicle  of  the  cat,  rabbit,  dog, 
or  rat,  are  hardened  in  2-per-cent.  sol.  of  potassium  bichro- 
mate and  alcohol.  Sections  from  the  different  parts  are 
stained  double,  and  mounted  in  glycerin  or  balsam.  The 
differences  between  the  structure  of  the  testicle  in  the 
lower  animals  and  man  cannot  be  described  here. 

THE    PROSTATE    GLAND. 

The  prostate  is  a  racemose  gland,  in  which  the 
alveoli,  instead  of  being  more  or  less  spheroidal,  as 
is  usually  the  case  in  racemose  glands,  are  often 
very  much  elongated  and  irregular  in  shape,  and 
very  frequently  present  high,  narrow,  irregular  folds 
in  their  walls.  The  alveoli  are  lined  for  the  most 
part  with  a  single  layer  of  cylindrical  epithelium. 
The  excretory  ducts  are  lined,  within  the  gland,  with 
cylindrical,  which  pass  over  into  flattened  cells,  as 
they  approach  the  urethral  orifice.  The  alveoli  and 
ducts  lie  embedded  in  a  dense  mass  of  interlacing 


THE  GEXERATIVE  ORGANS.  1 93 

bundles  of  smooth  muscular  tissue,  intermingled 
with  elastic  fibres  and  a  small  amount  of  fibrillar 
connective  tissue.  In  the  periphery  of  the  gland 
the  muscular  tissue  forms  a  sort  of  capsule  of  vary- 
ing thickness,  which,  in  turn,  is  enclosed  in  a  fibrous 
tunic. 

PRACTICAL    STUDY. 

Section  of  Gland. — A  bit  of  that  portion  of  the  pros- 
tate gland  of  man  which  lies  behind  the  urethra,  is 
hardened  in  potassium  bichromate  and  alcohol,  and  the 
sections  are  stained  double  and  mounted  in  balsam. 

URETHRA    AND    CORPUS    SPONGIOSUM. 

The  tissues  which  form  the  penis  are  so  various, 
and  present  such  essential  differences  in  their  ar- 
rangement in  different  parts  of  the  organ,  that  a  de- 
tailed study  of  its  structure  does  not  lie  within  the 
scope  of  this  manual.  Inasmuch,  however,  as  the 
urethra  is  so  frequently  the  seat  of  surgical  opera- 
tions, and  as  the  corpus  spongiosum,  which  sur- 
rounds a  portion  of  it,  presents  an  example  of  a 
variety  of  tissue  which  we  shall  have  no  opportunity 
to  study  elsewhere,  it  is  desirable  to  briefly  consider 
their  structure  here. 

The  urethra,  divided  into  three  portions — a  pros- 
tatic, a  membranous,  and  a  spongy, — consists  of  a 
mucous  membrane  which  is  surrounded  by  a  muscu- 
lar sheath.  The  considerable  differences  in  struc- 
ture which  its  different  parts  present  are  due  chieflv 


194  NORMAL  HISTOLOGY. 

to  variations  in  the  form  of  the  epithelium  and  the 
character  and  arrangement  of  the  muscular  tissue. 
The  mucosa  is  composed,  in  all  parts  alike,  of 
fibrillar  connective  tissue  containing  numerous  elas- 
tic fibres  and  richly  furnished  with  cells ;  upon  the 
free  surface  of  this  rests  the  epithelium,  which,  at 
the  meatus  and  in  the  fossa  navicularis,  is  of  the  flat, 
laminated  variety ;  this  passes  over  into  a  single 
row  of  cylindrical  cells  which  line  the  spongy  por- 
tion ;  and  this  in  turn  merges  into  the  spheroidal, 
pear-shaped,  prismatic,  and  flattened  cells  of  the 
prostatic  portion,  which,  lying  in  several  layers,  are 
quite  similar  in  form  and  arrangement  to  the  epithe- 
lium of  the  bladder. 

Outside  of  the  mucosa,  and  but  indistinctly  sep- 
arated from  it,  is  the  submucosa,  which  consists  of 
connective  tissue  with  elastic  fibres,  and  is  especially 
characterized  by  a  dense  net-work  of  veins  which  re- 
ceive their  blood  from  the  rich  capillary  system  lying 
in  the  mucosa  beneath  the  epithelium.  This  venous 
net-work  is  so  dense  and  the  vessels  so  large  as  to 
lend  to  the  submucosa,  especially  in  the  prostatic 
and  membranous  portions,  the  character  of  erectile 
tissue  (see  below). 

Here  and  there,  in  the  spongy  portion,  larger  and 
smaller  irregular  depressions,  called  lacuna  Morgagni, 
are  seen  in  the  surface  of  the  mucous  membrane. 
Racemose  glands,  called  Littres  glands,  lie  embed- 
ded in  the  -mucous  membrane,  sometimes  extending 


THE  GEXERATIVE  ORGANS.  1 95 

into  the  muscular  tunic  ;  their  excretory  ducts, 
sometimes  short,  sometimes  long  and  tortuous,  open 
on  the  surface  of  the  mucous  membrane.  In  the 
prostatic  portion,  the  prostatic  and  ejaculatory  ducts 
pierce  the  mucous  membrane,  as  do  the  ducts  of 
Cowper's  glands  that  of  the  posterior  segment  of  the 
spongy  portion. 

The  muscular  tunic  of  the  urethra  consists,  in 
general,  of  an  inner  longitudinal  and  an  outer  circu- 
lar layer  of  smooth  muscle-cells,  but  varies  greatly 
in  structure  in  the  different  portions.  The  outer 
layers  of  the  posterior  portion  of  the  canal  are 
formed,  in  part,  of  striated  muscle.  The  musculosa 
of  the  spongy  portion  consists  entirely  of  smooth 
muscle-cells,  and  forms  a  complete  circular  layer  in 
the  posterior  region  alone,  while  anteriorly  only 
scattered,  transversely  and  obliquely  placed  cells  are 
found. 

When  the  urethra  is  closed  the  mucous  membrane 
is  thrown  into  irregular  longitudinal  folds. 

The  urethra  is  enclosed  through  a  part  of  its 
length  in  the  corpus  spongiosum,  which  is  largely 
composed  of  a  kind  of  tissue  called  erectile  tissue ; 
and  before  describing  this  body  it  will  be  well  to 
consider  for  a  moment  the  nature  of  this  kind  of 
tissue. 

Erectile  tissue,  in  certain  cases,  consists  simply  of 
a  somewhat  circumscribed  collection  of  larger  and 
smaller  veins,   which  under,  certain    circumstances, 


I96  NORMAL  HISTOLOGY. 

may  become  distended  with  blood,  thus  causing  the 
part  in  which  they  lie  to  expand  ;  in  other  cases  it 
consists  of  numerous  larger  and  smaller  irregular- 
communicating  cavities,  separated  from  one  another 
by  broad  or  narrow  interlacing  bundles  of  connective 
tissue  and  smooth  muscular  tissue,  and  in  communi- 
cation with  arterial  trunks  or  capillary  blood-vessels  ; 
the  cavities  are  lined  with  endothelium,  and,  while 
usually  containing  but  a  small  amount  of  blood,  and 
in  this  condition  appearing  simply  as  slits  or  narrow 
irregular  spaces  in  the  tissue,  they  may,  under  cer- 
tain conditions,  become  distended  with  blood,  when 
they  assume  the  character  of  spheroidal,  or  broad 
elongated  cavities,  and  thus  cause  a  considerable 
increase  in  volume  of  the  part  in  which  they  are 
situated.  The  corpus  spongiosum  is  composed 
largely  of  erectile  tissue  of  the  character  last  de- 
scribed. 

The  corpus  spongiosum  is  enclosed  in  a  dense  con- 
nective-tissue sheath  which  contains  elastic  fibres 
and  a  few  smooth  muscle-cells.  From  this  a  multi- 
tude of  narrow  trabecular  or  septa,  composed  largely 
of  smooth  muscle-tissue,  pass  inward  in  various 
directions,  and  dividing  and  subdividing,  form  an 
intricate  series  of  spaces,  which  are  the  above-men- 
tioned blood-cavities.  This  system  of  trabecular  and 
septa  is  continuous,  within,  with  the  submucosa  of 
the  urethra. 

The  erectile  tissue   is  most  abundant   below  the 


THE  GENERATIVE  ORGANS.  1 97 

urethra ;  above,  the  blood  is  collected  into  venous 
trunks,  which,  joining  similar  trunks  from  the  cor- 
pora cavernosa  above,  convey  the  blood  away  from 
the  part.  The  corpora  cavernosa  are  similar  in 
structure  to  the  corpus  spongiosum. 

PRACTICAL   STUDY. 

Transverse  Sections. — The  corpus  spongiosum  and  the 
posterior  portions  of  the  urethra  are  dissected  from  the 
remainder  of  the  penis  and  hardened  in  potassium  bichro- 
mate and  alcohol.  Transverse  sections  from  the  different 
regions  are  stained  double  and  mounted  in  balsam, 

FEMALE  GENERATIVE  ORGANS. 
THE  OVARY. 

The  ovary  is  a  gland  whose  vesicular  alveoli  have 
no  excretory  ducts,  but  discharge  their  contents  by 
periodic  rupture  at  the  surface  of  the  organ.  These 
alveoli  are  called  Graafian  follicles,  and  the  ovum 
may  be  regarded  as  their  specific  secretion.  They 
lie  embedded  in  a  peculiar  connective-tissue  stroma, 
the  mterstitial  tissue,  and  in  the  adult  are  confined 
to  the  peripheral  zone.  We  have,  then,  in  studying 
the  ovary  to  consider  the  interstitial  tissue,  with  its 
blood-vessels  ;  and  the  glandular  tissue  or  Graafian 
follicles. 

On  looking  at  a  thin  transverse  section  of  the 
ovary,  we  see  that  it  presents  two  indistinctly 
separated  zones  ;  an  inner  or  central  zone,  including 


198  NORMAL  HISTOLOGY. 

the  ramifications  of  the  larger  blood-vessels,  and  called 
the  medullary  portion  ;  and  an  outer,  denser  cortical 
zone,  in  which  the  follicles  lie.  The  interstitial  tissue 
consists,  throughout  the  entire  organ,  of  connective- 
tissue  bundles,  which  in  the  medullary  portion  are 
somewhat  loosely  interwoven  and  associated  with 
elastic  fibres  and  smooth  muscle-cells.  In  the  corti- 
cal zone  the  muscular  elements  fail,  the  connective- 
tissue  bundles  are  finer,  and  cross  and  interlace  in 
all  directions,  surrounding  the  follicles,  in  whose 
vicinity  they  are  modified  so  as  to  form  a  kind  of 
capsule.  Near  the  surface  of  the  organ  the  connec- 
tive-tissue fibres  arrange  themselves  in  crossing  lay- 
ers, to  form  a  dense  but  not  distinct  sheath,  called 
the  albuginea. 

The  connective  tissue  of  the  ovary,  especially  in 
the  cortical  zone,  contains  a  great  number,  and  in- 
deed in  some  parts  seems  almost  entirely  to  consist 
of  spindle-shaped,  flattened,  and  irregular  branch- 
ing cells,  among  which  are  not  infrequently  larger 
and  smaller  pigmented  cells  and  polyhedral  cells 
resembling  epithelium. 

The  surface  of  the  organ  is  covered  by  a  single 
layer  of  cylindrical  epithelium,  whose  extreme  sig- 
nificance we  shall  recognize  when  we  study  the  de- 
velopment of  the  Graafian  follicles.  The  ovary  has 
no  serous  covering,  the  peritoneum  being  replaced 
by  the  epithelial  cells. 

The  interstitial   tissue  of  the  ovary  is  very  vascu- 


THE  GENERATIVE  ORGANS.  1 99 

lar;  the  large  arterial  trunks,  entering  from  the 
broad  ligament,  divide  and  subdivide  in  the  medul- 
lary portion,  and  pass  off  in  larger  and  smaller  twigs 
into  the  cortex,  from  which  an  abundant  capillary 
net-work  is  formed  around  the  follicles.  The  ar- 
teries frequently  run  a  very  tortuous  course,  twist- 
ing and  turning  .upon  themselves,  and  are  character- 
ized, moreover,  by  the  great  abundance  of  smooth 
muscle-cells  in  their  walls.  Lymphatic  vessels  are 
abundant.  Concerning  the  distribution  of  nerves, 
our  knowledge  is  still  very  meagre. 

We  have  now  to  consider  the  structure  of  the 
Graafian  follicles,  to  which  the  parts  just  described 
are  subservient.  The  Graafian  follicles  in  the  ovary 
of  the  adult  female,  at  the  child-bearing  age,  present 
by  no  means  the  same  appearance  ;  some  are  large 
and  readily  visible  to  the  naked  eye  ;  others  very 
minute,  and  presenting  under  the  microscope  an  en- 
tirely different  structure.  These  differences  in 
structure  are  accounted  for  by  the  fact  that  certain 
of  the  follicles  are  mature  or  approaching  maturity, 
while  others  are  still  undergoing*development. 

We  will  first  consider  the  structure  of  those  which 
are  mature,  or  nearly  so.  The  interstitial  tissue  of 
the  ovary  arranges  itself,  in  the  immediate  vicinity 
of  the  follicle,  in  the  form  of  a  tolerably  well-defined 
wall,  called  the  tJieca  folliculi,  consisting  of  an  ex- 
ternal denser  layer  and  an  internal  layer  very  richly 
supplied    with    cells,  and  containing   an   abundant 


200  NORMAL  HISTOLOGY. 

capillar}'  net-work.  Within  the  theca  folliculi  is  a 
layer,  usually  several  cells  deep,  of  larger  and 
smaller  spheroidal,  or,  in  the  periphery,  cuboidal 
epithelium,  called  follicular  epithelium,  and  consti- 
tuting the  so-called  membrana  granulosa.  At  one 
or  other  side  the  follicular  epithelium  is  heaped  up 
into  a  larger  or  smaller  mass,  which  projects  into 
the  cavity  of  the  follicle  and  contains  the  ovum  ; 
this  accumulation  of  cells  is  called  cumulus proligcrus 
or  germ-hill.  The  remainder  of  the  follicular  cav- 
ity  is  filled  with  a  colorless  fluid,  in  which  not 
infrequently  fine  granules  are  suspended.  This  fluid 
increases  in  quantity  as  the  follicle  approaches  ma- 
turity, and  the  pressure  occasioned  by  its  accumu- 
lation probably  conduces  in  no  slight  degree  to 
the  final  rupture  of  the  follicle.  The  follicular 
epithelium  immediately  surrounding  the  ovum  is 
cylindrical  and  arranged  radially  about  it. 

In  the  ovum  itself,  which  is  a  cell  of  the  most 
highly  developed  type,  we  recognize  four  structural 
elements:  I,  a  thick  hyaline  membrane,  presenting, 
with  high  powersr  a  delicate  radial  striation,  and 
called  the  zona  pellucida ;  2,  within  the  zona  pellu- 
cida  is  the  cell-body,  consisting  of  coarsely  and 
finely  granular  protoplasm,  and  usually  called  the 
vitellus  ;  3,  the  vitellus  encloses  a  comparatively 
large  vesicular,  transparent,  and  sharply-outlined 
nucleus,  the  germinal  reside ;  4,  the  germinal  vesicle, 
which  is  usually  somewhat  eccentrically  placed,  con- 


THE  GENERATIVE  ORGANS.  201 

tains,  in  addition  to  a  nearly  transparent  fluid,  a 
small  dark,  often  almost  opaque  nucleolus,  the  ger- 
minative  spot. 

As  the  Graafian  follicles  mature,  they  approach  the 
surface  of  the  ovary,  often  projecting  above  it  ;  the 
walls  become  thinner  and  less  vascular  at  the  pro- 
jecting side,  and  finally  burst  at  a  menstrual  period. 
The  ovum,  with  the  fluid  and  a  portion  of  the  follic- 
ular   epithelium,    is    discharged,    and    through    the 
hemorrhage  which  occurs  from  the  capillaries  in  the 
follicular  wall,  the  cavity  becomes  filled  with  blood. 
Changes  now  occur  in  the  walls  and  cavity  of   the 
follicle,  which  result   sooner  or  later  in  cicatrization 
and  obliteration  of  the  cavity.     These  changes  vary 
considerably,  depending  upon  whether  or  not  the 
ovum  is  impregnated  and  develops ;  and  the  differ- 
ence expresses   itself  chiefly  in  a  difference   in  size 
and  persistence  of   the  mass  of  tissue  called  the  cor- 
pus luteum,which  is  produced  by  a  growth  of  certain 
of  the  cells  which  remain  after  the  rupture  of  the 
follicle.     These    differences    cannot    be    considered 
here.     Tl\e  process  of  formation  and  disappearance 
of   the  corpus  luteum,   under  all  circumstances,   is 
essentially  the  following  :  The  blood  which  is  poured 
out  into  the  cavity  passes  through  the  same  retro- 
gressive  metamorphosis  which  extravasated   blood 
in  any  part  of  the  body  may  undergo  :  it  coagulates, 
the  serum  is  absorbed,  the  red  cells  disintegrate,  and 
the  coloring  matter  is  in  part  taken  up  by  su.-round- 


202  NORMAL  HISTOLOGY. 

ing  tissues,  in  part  transformed  into  yellowish  or  red 
hematoidin  (bilirubin)  crystals,  which  in  turn  may 
change  into  a  dark  brown  or  black  pigment,  and  be 
taken  up  by  surrounding  tissues,  or  remain  for  a  long 
time  unchanged.  Hand  in  hand  with  these  changes 
in  the  extravasated  blood,  go  important  changes  in 
the  follicular  epithelium  which  is  left  behind,  and  in 
the  cells  of  the  theca  folliculi.  These  cells  prolifer- 
ate and  form  a  soft,  yellowish,  very  vascular  tissue, 
resembling  mucous  tissue,  which  presently  under- 
goes fatty  degeneration.  This  yellow  mass,  sur- 
rounding and  enclosing  the  remains  of  the  extrava- 
sated  blood,  constitutes  the  corpus  luteum  ;  and,  as 
it  disappears,  its  place  is  occupied  by  firm,  dense 
connective  tissue,  which  usually  persists  for  a  long 
time  in  the  form  of  an  irregular  cicatrix,  whose  cells 
not  infrequently  still  contain  yellow  or  brown  or 
black  pigment. 

In  order  to  understand  all  the  forms  which  the 
immature  Graafian  follicles  present,  it  will  be  neces- 
sary to  study  the  way  in  which  these  structures 
originate.  They  are  produced  from  the  cylindrical 
epithelium  which  covers  the  ovary  and  is  called 
gcrm-epitJiclium.  At  an  early  period  of  life  the  con- 
nective-tissue stroma  of  the  ovary  grows  rapidly,  and 
partially  encloses  groups  of  the  germinal  epithelial 
cells,  which  themselves  proliferate,  and  dip  down 
into  the  stroma  of  the  organ  in  the  form  of  elon- 
gated,   solid    or   tubular    masses.      Presently    these 


THE  GENERA  TIVE  ORGANS.  203 

groups  of  cells  become  separated  from  the  germinal 
epithelium  at  the  surface,  and  appear  then  as  irregu- 
lar masses  of  polyhedral  or  spheroidal  cells,  enclosed 
on  all  sides  by  connective  tissue.  A  still  further 
growth  of  the  stroma  separates  these  masses  of  cells 
into  smaller  groups,  which  gradually  sink  farther 
from  the  surface  of  the  organ  and  become  more 
widely  separated  from  one  another,  while  new 
masses  of  germ-epithelium  are  being  enclosed  at  the 
surface.  The  cells  which  now  lie  in  these  separated 
cavities  may  at  first  all  look  alike,  but  later  we 
usually  find  that  one  of  them  is  larger  than  the 
others,  is  spheroidal,  and  occupies  the  centre  of  the 
cavity,  while  the  rest  are  more  or  less  cuboidal  and 
arranged  in  a  single  layer  around  the  wall,  closely 
enclosing  the  central  cell.  This  is  the  young 
Graafian  follicle,  and  the  central  cell  is  the  ovum  ; 
but  it  presents  as  yet  no  zona  pellucida.  Gradually 
the  follicular  epithelium  increases  in  quantity,  form- 
ing several  layers,  and  the  ovum  becomes  eccentri- 
cally placed.  There  is  still  no  cavity,  but  this  soon 
appears,  at  first  as  a  slit  between  the  cells,  and  then 
grows  wider  and  wider  as  fluid  accumulates  within 
it.  Changes  in  the  interstitial  tissue  lead  to  the  for- 
mation of  the  theca  folliculi,  the  epithelium  directly 
about  the  ovum  assumes  a  radial  arrangement,  the 
zona  pellucida  is  formed,  and  we  thus  have  the 
structure  of  the  maturing  follicle,  with  which  we 
are  already  acquainted. 


204  NORMAL  HISTOLOGY. 

PRACTICAL    STUDY. 

Section  of  Adult  Ovary. — The  human  ovary,  or  that  of 
a  recently  killed  dog  or  cat,  should  be  divided  transversely, 
great  care  being  taken  not  to  rub  the  surface,  since  the 
germ-epithelium  easily  comes  off,  and  placed  in  a  mixture 
of  equal  parts  of  one-quarter-per-cent.  chromic  acid  so- 
lution and  alcohol.  After  a  week  it  is  to  be  transferred 
to  alcohol,  in  which  in  a  few  days  it  will  become  suffi- 
ciently hard.  A  half  is  embedded  in  wax,  the  sections 
stained  double  and  mounted  in  balsam. 

Section  of  Developing  Ovary. — The  ovary  of  a  fcetal  or 
new-born  animal  is  hardened  and  prepared  as  above. 

THE    UTERUS. 

The  walls  of  the  uterus  consist  of  crossing  and 
interlacing  bundles  of  smooth  muscle-cells,  with  a 
small  amount  of  connective  tissue  enclosing  them 
and  binding  them  together.  The  muscular  tissue  is 
arranged  in  three  ill-denned  layers,  of  which  the 
middle  is  the  thicker.  It  may  be  said,  in  general, 
that  the  bundles  of  the  inner  layer  run  transversely 
around  the  organ,  those  of  the  middle  layer  are 
longitudinal,  while  those  of  the  outer  layer  are  quite 
irregular  ;  still  in  all  the  layers  there  is  great  lack  of 
uniformity  in  the  direction  of  the  cells.  A  part  of 
the  external  surface  of  the  uterus  is  covered  by  the 
peritoneum,  while  its  inner  surface  is  lined  with  a 
mucous  membrane.  The  latter  consists  of  a  frame- 
work formed  of  a  delicate  net-work  of  fibres,  between 


THE  GENERATIVE  ORGANS.  20$ 

which  lie  a  great  number  of  spheroidal,  fusiform,  an  ' 
branched  cells,  and  is  covered  on  the  free  surface  by 
cylindrical  ciliated  cells.  In  the  mucosa  the  simpl: 
or  branched  tubular  uterine  glands  are  embedded  ; 
they  are  often  tortuous,  and,  like  the  surface  of  the 
mucous  membrane,  are  lined  with  cylindrical  ciliated 
epithelium. 

The  surface  of  the  mucous  membrane  of  the  body 
of  the  uterus  is  smooth,  but  in  the  cervix  it  presents 
regular  folds,  the  so-called  plica  palmatce ;  the  con- 
nective-tissue framework  of  the  cervical  mucous 
membrane  is,  moreover,  firmer  in  texture,  contains 
fewer  glands,  and  these,  for  the  most  part,  are  more 
or  less  ^lobular  and  lined  with  short  cvlindrical  or 
cuboidal  epithelium.  The  ciliated  epithelium  of 
the  body  extends  over  on  to  the  mucous  membrane 
of  the  cervix,  where  it  becomes  continuous  with  the 
laminated  epithelium  covering  the  lower  portion  of 
the  canal  and  the  portio  vaginalis.  The  uterus  is  a 
very  vascular  organ  :  the  mucous  membrane  is  sup- 
plied with  a  rich  capillary  plexus,  which  passes  inward 
close  beneath  the  surface-epithelium. 

During  menstruation  the  mucous  membrane  be- 
comes thickened — partly  owing  to  the  engorgement 
of  its  blood-vessels,  and  partly  to  the  accumulation 
of  fluid  and  lymph-cells  in  its  interstices.  The 
uterine  glands  are  enlarged,  and  their  epithelium, 
as  well  as  that  of  the  general  surface  of  the  cavity, 
is  swollen.     To  what  extent  the  blood,  which,  mixed 


206  NORMAL  HISTOLOGY. 

with  mucus  and  separated  epithelium,  is  present 
in  the  cavity  at  the  menstrual  period,  is  due  to 
the  rupture  of  the  engorged  capillaries,  and  to 
what  extent  to  diapedesis,  is  not  yet  definitely 
determined. 

PRACTICAL   STUDY. 

Section  of  Human  Uterus. — The  uterus  is  cut  into  sev- 
eral pieces,  and  hardened  in  potassium  bichromate  and 
alcohol.  The  organ  should  be  procured  as  soon  as  pos- 
sible after  death,  since  the  cilia  of  the  lining  epithelium 
are  easily  destroyed  by  the  decomposition  which  com- 
mences very  early  in  the  uterine  cavity.  Sections  per- 
pendicular to  the  surface  of  the  mucous  membrane  are 
made  from  the  cervix,  and,  in  the  body,  from  the  lower 
portions  or  from  the  vicinity  of  the  entrance  of  the  Fal- 
lopian tubes,  since  here  the  uterine  glands  are  more  uni- 
formlv  arranged  at  right  angles  to  the  surface.  They  are 
stained  double  and  mounted  in  balsam. 

THE    VAGINA. 

In  the  walls  of  the  vagina  we  recognize  three  lay- 
ers :  an  outer  fibrous  layer,  a  middle  muscular  layer, 
and  a  lining  mucous  membrane. 

The  fibrous  layer,  by  means  of  which  the  vagina 
is  connected  with  adjacent  parts,  consists  of  connec- 
tive tissue  with  elastic  fibres,  and  in  texture  is  looser 
in  its  outer,  denser  in  its  inner,  portions.  The  mus- 
cular layer  consists  of  bundles  of  smooth  muscle- 
cells,  which  present  an  indistinct  grouping  into  an 


THE  GENERA  TIVE  ORGANS.  2QJ 

outer  portion,  formed  of  longitudinally  arranged 
cells,  and  an  inner,  in  which  they  have,  in  general,  a 
transverse  arrangement. 

The  mucous  membrane  consists  of  delicate  connec- 
tive tissue,  loose  in  texture  and  containing  coarser 
and  finer  elastic  fibres  ;  it  presents  numerous  trans- 
verse folds  and  elevations,  and  is  covered  by  lami- 
nated epithelium,  the  cells  in  the  lower  layers  being 
more  or  less  spheroidal,  but  flat  and  scale-like  at  the 
surface.  Numerous  papillae  project  into  the  epi- 
thelium. Venous  plexuses  are  formed  within  the 
deeper  portions  of  the  mucous  membrane,  which 
give  to  some  parts  of  that  structure  the  character  of 
erectile  tissue. 

PRACTICAL   STUDY. 

Transverse  Sections.— -A.  bit  of  the  vaginal  wall  should 
be  stretched  on  cork  and  placed  in  a  mixture  of  equal 
parts  of  one-fourth-per-cent.  solution  of  chromic  acid  and 
alcohol,  and  after  five  days  ^transferred  to  alcohol,  in 
which  the  hardening  is  completed.  Sections  at  right  an- 
gles to  the  surface  are  stained  double  and  mounted  in 
glycerin. 

Isolated  Surface-ceils. — From  a  bit  of  vagina  hardened 
as  above,  or  simply  in  alcohol,  the  cells  from  the  surface 
of  the  mucous  membrane  are  scraped  with  a  scalpel, 
stained  on  a  slide  with  hematoxylin,  and  mounted  in  glyc- 
erin. Familiarity  with  the  appearance  of  these  surface 
cells  is  of  practical  importance,  since  they  are  often  found 
in   urine. 


208  NORMAL  HISTOLOGY. 

MAMMARY    GLAND. 

The  mammary  gland  is  a  racemose,  and  when 
fully  developed  a  lobulated,  gland,  whose  spheroidal 
and  elongated  alveoli  are  formed  by  a  membrana 
propria  composed  of  flattened  cells,  and  lined  with 
cuboidal  epithelium.  The  excretory  ducts,  which 
in  each  gland  are  fifteen  to  twenty  in  number,  and 
open  at  the  surface  of  the  nipple,  are  lined  with 
cylindrical  epithelium.  Fibrillar  connective  tissue 
with  elastic  fibres  lies 'between  the  alveoli  and 
lobules,  and  is  abundantly  furnished  with  blood- 
vessels. 

The  gland  presents,  both  macroscopically  and  mi- 
croscopically, quite  marked  differences  in  appearance 
at  different  times,  depending  upon  age,  sex,  and,  in 
the  adult  female,  upon  whether  or  not  the  individ- 
ual is  pregnant,  and  upon  the  period  of  pregnancy 
and  the  occurrence  of  lactation. 

In  children,  as  in  the  adult  male,  the  gland  pre- 
sents, in  general,  a'  system  of  ramifying  ducts,  ter- 
minating in  blind  or  in  more  or  less  pouched  or 
dilated  extremities ;  these  lie  embedded  in  connec- 
tive tissue,  which  makes  up  the  greater  part  of  the 
bulk  of  the  gland.  At  puberty  the  gland  of  the  fe- 
male undergoes  considerable  development ;  well- 
defined  alveoli  are  found  in  the  periphery  of  the 
gland  connected  with  the  excretory  ducts,  but  the 
interstitial    tissue    is    still  more  abundant  than   the 


THE  GENERATIVE  ORGANS.  209 

gland-tissue  proper.  Essentially  in  this  condition 
the  gland  usually  remains  until  the  climacteric 
period,  if  pregnancy  does  not  occur. 

If,  however,  the  individual  becomes  pregnant,  the 
number  and  size  of  the  alveoli  rapidly  increase  ;  the 
ducts,  which  in  the  virgin  terminate  only  in  dilated 
extremities,  become  connected  with  extensive  groups 
of  newly  formed  alveoli ;  the  gland  assumes  a  tabu- 
lated character,  the  connective  tissue  between  the 
alveoli  is  very  much  less  abundant  in  proportion  to 
the  gland-tissue,  is  looser  in  texture,  and  contains  a 
great  number  of  larger  and  smaller  cells,  and  fre- 
quently fat-cells.  During  lactation  the  alveoli  are 
very  large,  their  epithelium  contains  fat-droplets  in 
considerable  number,  and  fat  is  found  in  greater  or 
less  quantity  in  the  dilated  cavities  of  the  alveoli, 
and  in  the  excretory  ducts.  When  the  secretory 
activity  of  the  gland  ceases,  it  undergoes  involution, 
the  alveoli  become  smaller,  fat  ceases  to  be  produced 
by  the  cells,  and  the  interstitial  connective  tissue 
becomes  proportionally  more  abundant. 

With  the  decline  of  the  reproductive  power  the 
mammary  gland  undergoes  a  marked  and  perma- 
nent involution:  the  terminal  alveoli  disappear, 
the  smaller  ducts  are  obliterated,  and  finally  little 
is  left  of  the  organ  excepting  the  deformed  and 
collapsed  larger  ducts  and  a  mass  of  connective 
tissue. 


210  NORMAL  HISTOLOGY. 

PRACTICAL   STUDY. 

Sections  of  Hardened  Gland. — Portions  of  the  gland 
from  the  human  subject  or  from  some  of  the  lower  mam- 
malia— if  possible,  both  in  a  condition  of  rest  and  of  func- 
tional activity — should  be  hardened  in  potassium  bichro- 
mate and  alcohol,  and  the  sections  stained  double  and 
mounted  in  glycerin  or  balsam. 


CHAPTER  XVI. 
THE  CENTRAL  NERVOUS  SYSTEM. 

THE    SPINAL    CORD. 

The  spinal  cord  contains  nerve-cells  and  nerve- 
fibres;  the  former  confined  to  the  central,  the  latter 
most  abundant  in  the  peripheral,  portions.  Th  . 
nerve-elements  lie  in  the  meshes  of  peculiarly 
arranged  connective  tissue  in  which  blood-vessels 
ramify,  and  the  whole  is  surrounded  by  a  vascular 
connective-tissue  membrane — the  pia  mater  spinalis. 
On  its  anterior  and  posterior  surfaces,  narrow  fis- 
sures, reaching  nearlv  to  the  centre,  divide  the  cord 
into  lateral  halves  ;  into  these  fissures,  called  the 
anterior  and  posterior  longitudinal  fissures,  the  pia 
mater  sends  membranous  prolongations.  The 
anterior  fissure  is  complete,  the  pia  being  found  on 
both  of  its  sides,  which  can  be  easily  separate  I 
while  in  the  posterior  fissure  the  sides  are  bound 
together  by  the  pia. 

In  transverse  sections  of  the  cord  a  central  gray 
portion  is  seen  which  is  surrounded  by  an  irregular 
white  zone.  The  form  which  the  gray  matter  as- 
sumes differs  considerably  in  different  parts  of  the 
cord,  but  has,  in  general,  the  form  of  an  unsymmet- 

211 


212  NORMAL  HISTOLOGY. 

rical  H  ;  the  cross-arm  of  the  H  is  formed,  in  great 
part,  by  the  nerve-substance  which  connects  the 
lateral  halves  of  the  cord,  and  is  called  the  gray 
commissure ;  the  uprights  of  the  H  lie  completely 
embedded  within  the  white  matter  of  the  lateral 
halves  ;  the  anterior  and  broader  ends  being  called 
the  anterior  comma,  the  posterior  and  narrow  ends 
the  posterior  comma.  Within  the  gray  commissure, 
and  separating  it  into  an  anterior  and  a  posterior 
portion,  a  narrow  canal,  called  the  central  canal, 
runs  the  entire  length  of  the  cord  ;  it  is  lined,  in 
early  life  at  least,  with  cylindrical  ciliated  cells,  and 
is  surrounded  by  delicate  connective  tissue.  From 
the  anterior  and  posterior  cornua  the  spinal  nerves 
pass  off,  dividing  the  white  matter  into  three  toler- 
ably distinct  portions  called  the  anterior,  lateral, 
and  posterior  columns. 

In  the  white  substance  of  the  cord  we  find  nerve- 
fibres,  connective  tissue,  and  blood-  and  lymph-ves- 
sels. The  fibres  are  for  the  most  part  medullated, 
but  have  not,  so  far  as  we  can  determine  with  the 
technical  facilities  at  present  at  our  disposal,  any 
neurilemma.  They  vary  greatly  in  diameter,  and 
although  a  large  majority  of  them  run  longitudi- 
nally for  the  greater  part  of  their  course,  we  find 
in  each  transverse  section  a  considerable  number 
which  run  in  an  oblique  or  horizontal  direction. 
Thus,  we  find  in  front  of  the  anterior  gray  commis- 
sure, at  the  bottom   of  the   anterior  fissure,  a  band 


THE  CEN1RAL  NERVOUS  SYSTEM.  2  I  3 

of  horizontally  arranged  fibres  running  from  one 
side  to  the  other,  called  the  zvhite  commissure.  The 
fibres  which  pass  out  of  the  gray  matter  to  form 
the  roots  of  the  spinal  nerves,  take  also  longitudi- 
nal and  oblique  courses. 

The  connective  tissue  of  the  white  substance  is 
intimately  connected  at  the  periphery  with  the  pia 
mater.  The  inner  layers  of  the  latter,  consisting 
of  delicate  fibrillated  and  a  few  elastic  fibres,  send 
into  the  cord  numerous  branching  septa,  which 
divide  the  space  occupied  by  the  nerve-substance 
into  smaller  irregular  spaces,  in  which  the  nerve- 
fibres  run,  these  being  surrounded  by  a  delicate 
net-work  of  fibrillse,  among  which,  here  and  there, 
variously  shaped,  for  the  most  part  flattened,  some- 
times branching,  connective-tissue  cells  are  found. 
This  delicate  supporting  connective  tissue  is  called 
neuroglia.  In  addition  to  the  fibrillar  and  cellular 
elements  above  described,  there  seems  to  be  a 
greater  or  less  amount  of  soft  homogeneous  or 
finely  granular  material  lying  in  the  interstices  of 
nerve-  and  connective-tissue  fibres.  The  amount  of 
connective  tissue  in  the  white  substance  of  the  cord, 
relative  to  the  nerve-fibres,  varies  considerably  in 
the  different  columns. 

In  the  gray  matter  of  the  cord,  we  have  also 
nerve-  and  connective-tissue  elements  and  blood- 
and  lymph-vessels  ;  the  first  consisting  of  ganglion- 
cells  and  nerve-fibres.     The  nerve-fibres  are  in   part 


214  NORMAL  HISTOLOGY. 

medullated,  in  part  naked  axis  cylinders  ;  and  be- 
sides these,  a  multitude  of  extremely  delicate  gray 
nerve-fibrils  occur,  which  seem  partly  to  come  from 
the  breaking  up  of  axis  cylinders,  and  partly  to  be  the 
delicate  branching  processes  of  ganglion-cells.  The 
nerve-cells  of  the  gray  matter  are,  for  the  most 
part,  multipolar,  and  vary  greatly  in  size,  the  largest 
being  found,  as  a  rule,  in  the  anterior  cornua.  They 
are  arranged  in  irregular  groups  in  different  parts 
of  the  cord. 

The  connective-tissue  elements  of  the  gray  mat- 
ter consist  of  a  delicate  neuroglia  framework,  simi- 
lar, in  most  respects,  to  that  supporting  the  nerve- 
fibres  between  the  coarser  septa  of  the  white 
substance.  In  the  hinder  portions  of  the  posterior 
cornua  there  is  a  circumscribed  area,  which,  in  the 
fresh  cord,  has  a  peculiar  gelatinous  appearance, 
and  is  called  the  substantia  gclatinosa  of  Rolando ; 
in  this  we  find  comparatively  few  nerve-elements 
and  much  connective  tissue.  The  limitations  of 
this  manual  will  not  permit  us  to  consider  more  in 
detail  what  is  known  of  the  course  of  the  nerve- 
fibres  through  the  cord,  and  the  more  exact  rela- 
tions to  the  ganglion-cells. 

The  blood-vessels  enter  the  cord  from  the  pia, 
along  the  septa  which  the  latter  sends  into  the 
organ,  and  ramify  in  its  substance,  the  capillary 
plexuses  being  denser  in  the  gray  than  in  the  white 
matter. 


THE  CENTRAL  NERVOUS  SYSTEM.  21  5 

THE    BRAIN. 

In  this  organ  also  we  have  gray  and  white  mat- 
ter;  but  they  are  arranged  in  a  much  more  compli- 
cated manner  than  in  the  spinal  cord,  the  grouping 
being  far  too  intricate  for  consideration  here.  The 
collections  of  gray  matter  are  variously  associated 
with  one  another  by  means  of  the  nerve-fibres  of  the 
white  substance. 

The  white  substance  of  both  cerebrum  and  cere- 
bellum consists  of  coarser  and  finer,  but  all  very 
small,  nerve-fibres,  running  in  various  directions,  and 
supported  by  a  delicate  connective-tissue  frame- 
work, similar  to  the  neuroglia  of  the  cord.  The 
gray  matter  consists  here,  as  in  the  cord,  of  gang- 
lion-cells, and  fine  gray  fibres  supported  by  connec- 
tive tissue.  In  parts  of  the  gray  as  of  the  white 
substance,  certain  cellular  and  fibrous  elements  oc- 
cur, of  which  it  is  at  present  impossible  to  say 
whether  they  are  connective  or  nerve-tissue.  In- 
deed, it  is  the  difficulty  of  determining  the  nature  of 
certain  structures  in  the  brain,  together  with  their 
extreme  delicacy,  and  the  difficulty  of  isolating 
them,  which  renders  the  histology  of  the  brain  so 
difficult  a  theme,  and  explains  the  unsatisfactory 
state  of  our  knowledge  concerning  it. 

We  cannot  do  more  in  these  lessons  than  to  study 
briefly  the  structure  of  two  of  the  best  known  and, 
at  present,   perhaps  most   interesting  parts  of  the 


2l6  NORMAL  HISTOLOGY. 

brain — namely,  the  cortical  portions  of  the  cere- 
brum and  cerebellum. 

a.  Cortex  of  the  Cerebrum. — The  structure  of  this 
part  of  the  brain  differs  somewhat  in  different  re- 
gions, but  that  of  one  of  the  frontal  lobes  is  suffi- 
ciently typical  for  our  purpose.  Here,  in  a  section 
perpendicular  to  the  surface,  and  extending  through 
the  entire  depth  of  the  gray  matter,  five  zones  or 
layers  may  be  recognized,  which,  however,  merge 
into  one  another. 

In  the  most  superficial  layer,  the  connective-tissue 
elements  preponderate,  and  among  them,  delicate 
nerve-fibrils  interlace,  and  a  few  small,  scattered 
globular  and  elongated  branching  nerve-cells  are 
found  ;  the  second  layer  is  characterized  by  a  great 
number  of  small  more  or  less  pyramidal  cells  ;  the 
third  and  broadest  layer  contains  a  proportionately 
smaller  number  of  ganglion-ceils  than  the  second, 
but  they  are  larger,  and,  for  the  most  part,  pyra- 
midal, or  broad  spindle-shaped,  and  multipolar,  with 
their  long  axes  perpendicular  to  the  surface  of  the 
cortex  ;  in  the  fourth  layer,  which  is  much  narrower 
than  the  last,  are  large  numbers  of  small  globular 
and  irregular-shaped  and  branching  cells ;  the  fifth 
layer,  finally,  contains  medium-sized  spindle-shaped 
cells,  with  long  tapering  processes,  together  with  a 
certain  number  of  smaller  irregular-shaped  cells.  In 
the  third  layer,  certain  of  the  delicate  nerve-fibres 
begin    to  take   a   more   regular  course  toward  the 


THE  CENTRAL  NERVOUS  SYSTEM.  217 

white  matter;  and  in  the  fourth  and  fifth  layers, 
they  are  readily  seen  in  distinct  bundles  passing  in- 
ward between  the  ganglion-cells. 

b.  Cortex  of  the  Cerebellum. — In  sections  through 
the  cortex  of  the  cerebellum  perpendicular  to  the 
surface,  three  distinct  layers  are  recognizable  :  the 
outer,  sometimes  called  the  molecular  layer,  consists, 
like  the  outer  layer  of  the  cerebral  cortex,  of  a  deli- 
cate connective-tissue  framework,  which  supports 
fine  nerve-fibres  and  small  spindle-shaped  and 
branching  nerve-cells  ;  the  middle,  cellular  layer,  is 
formed  by  an  irregular  row  of  large  ganglion  cells, 
Purkinjes  cells,  whose  branching  processes  extend 
into  and  ramify  in  the  outer  layer,  while  the  axis- 
cylinder  process  passes  inward  through  the  inner 
layer;  the  inner,  granular  layer,  contains  a  great 
number  of  small  spheroidal  cells  whose  nature  is  un- 
determined. The  granular  layer  merges  gradually 
into  the  white  substance  and  is  thickest  at  the  sum- 
mit of  the  convolutions,  where  also  Purkinje's  cells 
are  most  abundant. 

The  blood-vessels  penetrate  the  cortex  of  both 
cerebrum  and  cerebellum,  in  the  form  of  small  arte- 
rial twigs  from  the  pia,  and  form  an  abundant  net- 
work in  the  gray  substance,  being  somewhat  differ- 
ently distributed  in  different  parts  of  the  organ,  and 
less  abundant  in  the  white  chan  in  the  gray  matter. 

The  Dura  Mater  of  the  brain  is  a  dense  connective- 
tissue  membrane  containing  numerous  elastic  fibres 
and  lined  within  by  endothelial  cells.     Where  it  is 


213  NORMAL  HISTOLOGY. 

attached  to  the  bones  of  the  skull,  to  whose  inner 
surface  it  acts  as  periosteum,  the  tissue  on  the  at- 
tached surface  is  looser  in  texture  and  abundantly- 
supplied  with  blood-vessels.  It  contains  the  ordi- 
nary flattened  connective-tissue  cells,  and  usually  a 
certain  number  of  i;  plasma  "  cells.  The  dura  mater 
of  the  cord  does  not  form  the  periosteum  of  the 
bones  forming  the  spinal  canal,  and  hence  in  it  the 
looser  vascular  layer  is  for  the  most  part  wanting. 

The  Pia  Mater  of  the  brain  is  a  thin  connective- 
tissue  membrane,  covered  on  its  outer  surface  with 
endothelium,  and  containing  an  exceedingly  abun- 
dant net-work  of  blood-  and  lymph-vessels.  Over 
the  surface  of  the  convolutions  it  forms  a  single  mem- 
brane containing  numerous  small  lymph-sinuses  ; 
but  as  it  approaches  the  sulci  it  is  partially  separated 
into  two  indistinct  lavers,  the  outer  bridging  over 
the  sulci,  while  the  inner  and  more  vascular  laver 
dips  down  to  the  bottom  of  them.  The  space 
within  the  sulci,  between  the  two  layers,  is  occupied 
by  numerous  larger  and  smaller  lymph-sinuses, 
which  under  normal  conditions  are  little  more  than 
slits  in  the  connective  tissue,  lined  with  endothelium  ; 
but  they  are  capable  of  considerable  dilatation  when, 
for  any  reason,  fluid  accumulates  in  the  meshes  of 
the  pia.  These  spaces  are  called  sub-arachnoidal 
lympli-spaccs  ;  the  outer  layer  of  the  pia  having  been 
formerly  regarded  as  a  distinct  membrane,  and  called 
the  arachnoid.  These  sub-arachnoidal  lymph  spaces, 
as  well  as  the  other  numerous  lymph-channels  of  the 


THE  CENTRAL  NERVOUS   SYSTEM.  219 

pia.  are  in  communication  with  lymph-channels, 
called  perivascular  lymph-channels,  which  ensheath 
certain  of  the  blood-vessels  as  they  enter  the  brain- 
substance. 

PRACTICAL   STUDY. 

Transverse  Sections  of  Spinal  Cord. — A  perfectly  fresh 
human  cord  should  be  freed  from  its  dura  mater,  and  short 
segments  from  different  portions  of  it  hardened  in  Mtiller's 
fluid  and  alcohol.  Thin  transverse  sections  are  made 
through  the  entire  cord  in  different  poriions,  and  may  be 
stained  first  lightly  with  hematoxylin  and  then  deeply 
with  eosin  and  mounted  in  balsam.  A  very  excellent 
hardening  agent  for  the  nerve-centres  is  the  bichromate 
of  ammonium,  2-per-cent.  solution,  in  which  small  frag- 
ments should  lie  for  a  week,  and  then,  after  careful  wash- 
ing, the  hardening  completed  with  dilute  and  strong  al- 
cohol.    They  may  be  stained  and  mounted  as  above. 

Sections  of  Cortex  of  Cerebrum  and  Cerebellum. — These 
should  be  prepared  in  the  same  way  as  the  spinal  cord, 
pieces  from  the  cortex  of  the  frontal  lobes  being  taken, 
and  from  the  cortex  of  the  cerebellum  at  any  convenient 
part  ;  they  should  not-  be  larger  than  a  cubic  centimetre, 
and  the  chromic  fluid  well  washed  out  before  immersion 
in  alcohol. 

Dura  Mater. — A  bit  of  this  membrane  should  be 
stretched  on  a  piece  of  cork  with  pins,  hardened  in  alco- 
hol, embedded  in  hardened  liver,  and  thin  transverse 
sections  made,  stained  double  and  mounted  in  glycerin 
or  balsam. 

Pia  Mater. — This  may  be  prepared  by  the  method 
given  on  page  125. 


CHAPTER  XVII. 

THE  SKIN  AND  ITS  ADXEXA. 

THE    SKIX. 

We  recognize  in  the  skin  three  layers  of  tissue  : 
I,  an  outer,  epithelial  layer,  the  epidermis;  beneath 
this,  2,  a  layer  of  quite  firm  and  dense  connective 
tissue,  the  corium — true  skin,  cutis  vera,  or  derma ; 
3,  a  layer  of  looser  connective  tissue,  the  subcu- 
taneous tissue,  which,  merging  into  the  corium, 
serves  to  bind  it  to  the  underlying  parts.  The  skin 
is  variously  modified  in  structure  in  different  parts 
of  the  body,  corresponding  to  the  different  condi- 
tions of  exposure  and  wear  to  which  it  is  subjected, 
and  to  form  certain  supplementary  structures,  such 
as  the  hair,  nails,  etc.  :  and  contains  various  sensory 
and  secretory  structures. 

In  the  epidermis  we  recognize  two  tolerably  dis- 
tinct layers  of  cells:  I,  an  outer  or  Jiorny  layer,  con- 
sisting of  very  thin,  transparent,  tough,  scale-like 
cells,  which  present,  for  the  most  part,  no  nuclei, 
and  are  packed  closely  together ;  2,  an  inner  layer, 
the  so-called  mucous  or  MalpigJiian  layer,  consisting 
of  larger  and  smaller  nucleated  cells  of  varying 
shape   and    character:    in    the    deeper  portion,   ad- 

220 


THE  SKIN  AND  ITS  ADNEXA.  221 

joining  the  corium,  the  cells  are  more  or  less  cyl- 
indrical ;  above  this  they  are  spheroidal  or  polyhe- 
dral or  elongated  ;  still  nearer  the  surface  they  be- 
come flattened,  and  finally  merge  into  the  thin  cells 
of  the  horny  layer.  In  the  middle  zone  the  cells 
present  a  peculiar  jagged  outline,  looking  as  if  they 
were  bordered  by  short  delicate  spines,  by  which 
the  cells  appear  dove-tailed  together.  These  spined 
cells — called  prickle  cells — are  very  characteristic  of 
this  part  of  the  epidermis,  and  are  also  found  in 
certain  other  parts  of  the  body  where  stratified 
epithelium  occurs,  as  in  the  vagina,  mucous  mem- 
brane of  the  mouth,  etc. 

The  relative  thickness  of  the  horny  and  Mal- 
pighian  layers  of  the  epidermis  differs  greatly  in 
different  parts  of  the  body  ;  in  some  parts  of  the 
palms  of  the  hands  and  soles  of  the  feet  the  horny 
layer  is  very  thick,  and  here  we  often  find  that  the 
cells  which  lie  between  the  horny  and  Malpighian 
layers  form  a  distinct  narrow,  transparent  zone, 
called  the  stratum  lucidum.  The  deeper  cells  of  the 
Malpighian  layer  contain,  uniformly  in  the  negro, 
and  occasionally  in  circumscribed  regions  in  white 
men,  more  or  less  brown  or  black  pigment. 

The  epidermis  forms  in  but  few  regions  of  the 
body  a  layer  of  uniform  thickness,  since  the  corium 
sends  up  into  it,  at  varying  intervals,  simple  or 
branching,  variously  shaped  papillce,  the  valleys  be- 
tween  which,  as  well   as   their  summits,  being  cov- 


222  NORMAL  HISTOLOGY, 

ered  by  the  cells  of  the  Malpighian  layer.  If  we 
imagine  a  section  made  through  the  skin,  parallel 
with  its  surface,  and  just  deep  enough  to  cut  off  the 
tops  of  the  papillae,  the  cells  of  the  Malpighian 
layer  which  lie  between  the  latter,  would  appear,  on 
looking  at  the  cut  surface,  to  be  arranged  in  the 
form  of  a  net-work,  whose  meshes  are  filled  by  the 
papillae  of  the  corium.  Hence  it  is  that  these  col- 
lections of  cells  have  received  the  name  retc  Mai- 
pigJii. 

The  corium  is  formed  of  interlacing  bundles  of 
connective  tissue,  which  are  coarser  in  the  deeper, 
finer  in  the  more  superficial  portions,  where  they 
extend  into  the  epidermis,  forming  the  papillae. 
Embedded  in  the  papillae  are  capillary  blood-vessels, 
nerves,  and  special  terminal  nerve-apparatuses.  Elas- 
tic fibres  are  present  in  considerable  number,  and  in 
the  interstices  of  the  fibres  lie  flattened,  spindle- 
shaped,  branching,'  and  small  spheroidal  cells.  In 
addition  to  these  elements  we  sometimes  find  mus- 
cular tissue  in  the  corium  ;  thus  striated  muscular 
fibres  occur  in  certain  parts  of  the  skin  of  the  face  ; 
and  smooth  muscular  tissue,  aside  from  that  belong- 
ing to  the  hair-follicle,  is  found  about  the  sweat- 
glands,  in  the  skin  of  the  scrotum  and  penis,  and  in 
the  nipple  and  its  areola. 

The  subcutaneous  connective  tissue  we  have  al- 
ready studied  when  considering  the  connective  tissue 
in  detail.     In  some   parts  of   the  skin    its  texture  is 


THE  SKIN  AND  ITS  ADNEXA.  223 

so  loose  that  the  corium  and  epidermis  can  be 
readily  moved  to  and  fro  upon  the  underlying  parts 
or  pinched  up  in  folds ;  in  others  its  fibres  are  short 
and  tense,  and  bind  the  corium  closely  to  the  parts 
beneath.  In  the  subcutaneous  tissue  of  most  parts 
of  the  body,  greater  or  smaller  deposits  of  fat  occur, 
forming  the  panniculus  adiposus ;  but  in  the  subcu- 
taneous tissue  of  the  scrotum,  penis,  eyelids,  and  the 
pinna  of  the  ear,  fat  is  not  formed. 

Blood-vessels. — The  arteries  of  the  skin,  which 
enter  through  the  subcutaneous  tissue,  give  off,  in 
general,  three  sets  of  branches,  through  which  the 
blood  is  distributed  to  three  principal  sets  of  capil- 
laries :  first,  to  those  which  supply  the  fat-tissue ; 
second,  to  those  which  ramify  in  the  sweat-glands; 
third,  to  those  which  supply  the  hair-follicles,  se- 
baceous glands,  and  the  papillae  of  the  corium. 
Each  papilla  is  furnished  with  a  capillary  loop,  ex- 
cept when  it  contains  a  tactile  corpuscle,  when  the 
former  may  be  absent. 

THE    NAIL. 

We  recognize  in  the  hard  substance  of  the  nail, 
which  corresponds  to  the  horny  layer  of  the  epi- 
dermis, a  body  and  a  root ;  the  former  lies  upon  a  por- 
tion of  the  somewhat  modified  corium,  called  the 
nail-bed,  while  the  root  is  embedded  in  a  shallow 
pocket  of  skin,  the  corium  of  which  constitutes  the 
matrix  of  the  nail.     The   corium   of   the   nail   does 


224  NORMAL  HISTOLOGY. 

not  differ  essentially  from  that  of  the  skin  in  een- 
eral ;  it  is  intimately  connected  with  the  periosteum 
of  the  phalanx,  and  presents  longitudinal  ridges, 
low  in  the  matrix,  higher  in  the  nail-bed,  which  are 
covered  with  papillae.  The  latter  are  bent  obliquely 
forward,  and  are  more  abundant  in  the  nail-bed 
than  in  the  matrix.  Upon  and  between  the  pa- 
pillae several  layers  of  variously  shaped  celis  lie, 
corresponding  to  the  Malpighian  layer  of  the  skin. 
In  the  body  these  cells  pass  quite  abruptly  into  the 
flat,  horny,  nucleated  cells  of  the  hard  substance  of 
the  nail ;  in  the  matrix,  however,  the  transition  is 
very  gradual,  and  it  is  here  that  the  growth  of  the 
nail  occurs.  The  lunula  is  a  portion  of  the  nail  in 
which  the  Malpighian  layer  is  very  thick,  as  it  is  in 
all  parts  of  the  root,  and  being  evenly  distributed 
over  the  surface  of  the  papillae,  does  not  permit  the 
color  of  the  blood  in  the  capillaries  of  the  papillae 
to  be  seen,  as  it  is  in  the  rest  of  the  nail-bed,  where 
the  longitudinal  ridges  are  higher,  and  covered  by 
fewer  cells. 

THE    HAIR. 

We  distinguish  in  the  hair :  the  shaft,  which  pro- 
jects above  the  surface  of  the  skin  :  the  root,  which 
is  embedded  in  an  oblique  tubular  depression,  called 
the  follicle ;  and  the  bulb,  a  dilated  portion  at  the 
bottom  of  the  follicle  in  which  the  hair  ends.  The 
follicle   sometimes   extends    into   the   subcutaneous 


THE  SKIN  AND  ITS  ADNEXA.  225 

tissue,  sometimes  only  into  the  corium,  and  its  walls 
are  formed  in  the  first  place  by  a  sheath  of  connec- 
tive tissue  continuous  with  the  corium,  in  which 
numerous  blood-vessels  ramify  ;  this  sheath  is  lined 
by  a  thin  transparent  membrane,  called  the  vitreous 
membrane.  Within  this  follicular  wall  lies  the  root- 
sheath,  which  consists  of  two  layers :  an  outer 
thicker  layer,  formed  by  the  dipping  down  into  the 
follicle  of  the  cells  of  the  rete  Malpighi,  and  hence 
consisting  of  cylindrical,  spheroidal;  and  somewhat 
flattened  cells  ;  and  an  inner  layer  made  up  in  turn 
of  an  external  layer,  called  Henle ' s  sheath,  in  which 
the  cells  resemble  those  of  the  horny  layer  of  the 
epidermis,  and  are  closely  packed  together  to  form 
a  transparent  mass  ;  and  an  internal  layer,  called 
Huxley  s  sheath,  whose  cells,  belonging  more  prop- 
erly to  the  hair  itself,  are  irregularly  polygonal, 
somewhat  flattened,  and  contain  an  elongated  nu- 
cleus. 

Both  at  the  opening  of  the  follicle,  and  at  its 
base,  the  layers  of  the  root-sheath  become  indis- 
tinct, merging  on  the  one  hand  into  the  cells  of  the 
epidermis,  and  on  the  other  into  those  of  the  hair- 
bulb.  At  the  bottom  of  the  follicle  is  a  projection 
from  the  connective  tissue  forming  the  wall  of  the 
follicle,  in  the  form  of  a  papilla,  which  corresponds 
to  the  papillae  of  the  skin,  and  upon  which  the  hair- 
bulb  rests,  surrounding  it  at  the  top  and  sides. 
The  hair  is  produced   by  the  growth  of  cells  about 


226  NORMAL  HISTOLOGY. 

the  papilla;  directly  covering  the  latter  are  cylin- 
drical and  cuboidal  cells,  corresponding  to  those  of 
the  rete  Malpighi,  which  gradually  become  changed 
in  shape,  and  more  or  less  horny,  and  form  the  sub- 
stance of  the  hair-shaft. 

In  the  shaft  we  recognize  three  portions:  i.  A 
central  or  medullary  portion,  composed  of  cuboidal 
or  more  or  less  flattened  cells,  not  infrequently  en- 
closing between  them  tiny  bubbles  of  air,  which 
give  the  centre  of  the  hair  a  dark  appearance  by 
transmitted  light.  Outside  of  this  is  2,  the  cortical 
portion,  making  up  the  larger  part  of  the  bulk  of 
the  shaft,  and  composed  of  tough,  horny,  elongated, 
flattened  cells  closely  packed  together,  and  having 
within  and  between  them,  except  in  colorless  hairs, 
granules  of  variously  colored  pigment.  In  that 
portion  of  the  hair  which  lies  witliin  the  follicle, 
and  between  the  bulb  and  the  free  shaft,  we  find, 
while  the  hair  is  growing,  that  the  cells  of  the  cor- 
tical layer  are  larger,  less  flattened  and  horny,  and, 
as  above  mentioned,  merge  into  the  large  cylindrical 
and  spheroidal  cells  of  the  bulb.  Finally,  the  shaft 
is  covered,  3,  by  the  so-called  cuticula,  consisting  of 
thin  rectangular,  non-nucleated,  scale-like  cells, 
which  lap  over  one  another,  so  that  the  lower 
cells,  i.  c,  those  nearest  the  root  of  the  hair,  cover 
a  portion  of  the  cells  beyond,  and  these  free  edges 
often  project  slightly  from  the  surface  of  the  hair, 
giving  it  a  finely  serrated  appearance. 


THE  SKIN  AND  ITS  ADNEXA.  227 

SEBACEOUS    GLANDS. 

These  are  racemose  glands,  whose  excretory  ducts 
are  lined  with  polyhedral  and  somewhat  flattened 
cells.  The  alveoli,  bounded  by  a  membrana  propria, 
are  lined  with  granular  polygonal  epithelium,  and 
the  cavity  is  more  or  less  filled  with  larger  polyhed- 
ral cells,  crowded  with  fat-droplets.  The  sebaceous 
glands,  as  a  rule,  either  open  into  a  hair-follicle  near 
the  surface  of  the  skin,  or  their  excretory  ducts  are 
pierced  near  the  surface  by  the  shaft  of  a  hair. 

The  hair-follicle,  as  above  mentioned,  is  placed 
obliquely  in  the  skin,  and  at  the  side  at  which  it 
forms  an  oblique  angle  with  the  surface,  a  bundle 
of  smooth  muscle-cells  is  placed.  This  is  attached 
to  the  connective-tissue  sheath  of  the  follicle  in  its 
lower  third,  and,  passing  obliquely  upward,  is  in- 
serted into  the  upper  portion  of  the  corium  at  some 
distance  from  the  opening  of  the  follicle.  A  con- 
traction of  the  muscle  thus  placed  will,  of  course, 
draw  the  hair-follicle,  and  with  it  the  shaft,  into  a 
position  more  nearly  perpendicular  to  the  surface 
of  the  skin,  and  hence  it  is  called  the  erector  pilcu. 
As  a  rule,  the  sebaceous,  follicle  lies  above  the 
erector  muscle  in  the  angle  which  it  forms  with  the 
upper  portion  of  the  hair-fol!icle,  and  is  moved  with 
the  hair  when  the  muscle  contracts,  and  may  even 
be  pressed  upon  by  it,  when,  as  is  frequently  the 
case,  it  runs   closely  over  the  surface  of  the  gland. 


228  NORMAL  HISTOLOGY. 

This  relation  of  the  erector  pilae  muscle  to  the  se- 
baceous gland  is  probably  not  without  significance 
in  connection  with  the  discharge  of  the  secretion  of 
the  latter. 

SWEAT-GLANDS. 

The  sweat-glands,  which  are  found  in  the  skin  of 
almost   all  parts  of  the  body,  although  much   more 
abundant  in  some  than  in  others,  are  tubular  glands, 
the   tube   consisting  of   a  membrana   propria,  lined 
throughout  with  polyhedral  and  cuboidal  epithelium. 
Its  lower  extremity,  coiled  into  a  ball,  and  held  to- 
gether by  loose  connective  tissue,  lies  sometimes  in 
the   corium,  sometimes   in  the  subcutaneous  tissue. 
The  upper  portion  of  the  tube,  which  serves  as  the 
excretory   duct,  passes   to  the   surface  of  the  skin, 
often  taking  a  wavy   course    through  the    corium. 
It   pierces   the  epidermis  between   two  papillae,  and 
here  the  walls  of  the  duct  cease,  and  it  is  bordered 
by  epidermis-cells  alone.     If  the   epidermis-layer  is 
thick,  as   in  the  palm,  etc.,  the   course  of  the   duct 
through  it  is  a  remarkably  winding  one.     An  abun- 
dant capillary  net-work  lies  in  the  loose  connective 
tissue  of  the  gland-coil. 

NERVES. 

The  nerves  of  the  skin  ramify  in  the  subcutaneous 
tissue,  and  a  certain  number  of  them  terminate  here 
in   the   so-called   Pacinian  bodies ;  others  pass  into 


THE  SKIN  AND  ITS  AD  N  EX  A.  229 

the  corium,  where  they  form  plexuses,  varying  in 
character  in  different  parts  of  the  body.  From 
these,  certain  medullated  nerves  pass  into  the  papillae 
and  terminate  in  the  tactile  corpuscles  (called  Meiss- 
ner's  corpuscles) ;  others  pass  to  the  hair-follicles 
and  sebaceous  glands  ;  still  other,  non-medullated 
nerves  enter  the  papillae  or  pass  between  the  cells 
of  the  rete  Malpighi,  but  their  mode  of  termination 
is  not  yet  definitely  ascertained.  The  structure  of 
the  Pacinian  bodies  and  Meissner's  corpuscles  is  too 
intricate,  and  the  methods  required  for  their  com- 
plete demonstration  too  elaborate,  to  justify  their 
further  consideration  here. 

PRACTICAL   STUDY. 

Sectio?is  of  Injected  Skin. — A  piece  of  skin  is  removed 
from  an  injected  leg  or  arm,  care  being  taken  to  include 
the  subcutaneous  tissue  to  a  considerable  depth  ;  it  is 
stretched  flat  on  a  bit  of  cork,  and  placed  in  a  mixture  of 
equal  parts  of  one-half-per-cent.  chromic  acid  solution 
and  alcohol  ;  after  ten  days  it  is  transferred  to  strong  al- 
cohol. When  sufficiently  hard  it  is  embedded  in  wax  or 
liver,  and  sections  made  perpendicular  to  the  surface  ; 
these  are  stained  lightly  with  haematoxylin,  then  with 
eosin,  and  mounted  in  balsam. 

Sections  of  Skin  of  Negro. — The  skin  is  hardened  in  the 
chromic  acid  and  alcohol  mixture,  the  sections  stained 
with  picro-carmine,  and  mounted  in  glycerin. 

Sectio?is  of  the  Nail. — A  nail  should  be  separated  from 
a  finger  which  has  been  hardened  in   alcohol,  together 


23O  NORMAL  HISTOLOGY. 

with  as  much  as  possible  of  the  connective  tissue  which 
binds  it  to  the  bone.  In  order  to  render  the  soft  parts 
more  consistent  and  more  nearly  like  the  body  of  the  nail, 
so  that  uniformly  thin  sections  can  be  made,  the  whole 
should  be  hardened  by  the  gum  method;  see  page  115. 
Longitudinal  and  transverse  sections  are  made  through 
the  entire  nail,  and  stained  double  and  mounted  in 
balsam. 

Sections  of  Hairs  from  Skin  of  Scalp. — A  piece  of  skin 
from  the  scalp  of  an  adult  is  stretched  on  cork  and  hard- 
ened in  the  above  chromic  acid  mixture.  Sections  are 
made  as  nearly  as  possible  in  the  direction  of  the  hair- 
follicles,  so  as  to  include  the  entire  root  and  bulb,  and 
also  at  right  angles  to  them.  They  may  be  stained  double 
and  mounted  in  balsam  ;  or  instead  of  first  staining  with 
hematoxylin  they  may  be  laid  for  twenty-four  hours  in  a 
strong  aqueous  solution  of  analin  green  and  then  dehy- 
drated in  the  usual  way  in  the  eosin  alcohol.  By  the 
latter  method  a  very  brilliant  differentiation  in  color  in 
the  layers  of  the  inner  root-sheath  may  be  obtained. 

Cells  of  the  Cuticula  of  the  Hair. — These  are  readily 
seen  by  placing  a  hair  on  a  slide  in  a  drop  of  strong  sul- 
phuric acid,  and  covering  ;  in  a  short  time  the  cells  will 
have  become  so  much  loosened  that  if  the  cover  be  lightly 
tapped  upon,  a  certain  number  of  them  will  float  off  into 
the  acid,  or  may  be  seen  bristling  out  from  all  sides  of 
the  shaft. 

Section  of  Skin  from  the  Finger-tips. — This  is  prepared 
like  the  injected  skin,  and  will  show  the  thick  layer  of 
epidermis-cells  with  the  stratum  lucidum,  the  tortuous 
course  of  the  sweat-gland  ducts  through  the  epidermis. 


THE  SKIN  AND  ITS  ADNEXA.  23  I 

In  this  preparation  the  ovoidal  tactile  corpuscles  may  be 
seen  lying  in  some  of  the  papillae,  and  if  the  subcutaneous 
tissue  has  been  included  in  the  section  to  a  considerable 
depth,  transverse  or  longitudinal  sections  of  a  Pacinian 
body  may  be  found. 


CHAPTER  XVIII. 
THE  EYE. 

The  organ  of  sight  is  composed  of  the  eyeball 
and  various  accessory  structures,  such  as  the  eye- 
lids, lachrymal  gland,  muscles,  etc.  The  eyeball  is 
composed,  in  the  first  place,  of  a  dense,  firm,  sphe- 
roidal connective-tissue  envelope,  whose  anterior 
transparent  portion,  the  cor?iea,  is  more  convex  than 
the  posterior  opaque  segment,  the  sclerotic,  and 
differs  somewhat  from  it  in  structure;  the  sclerotic 
is  pierced  posteriorly  by  the  optic  nerve.  Within 
the  sclerotic  lies  a  vascular  tunic,  the  choroid, 
formed  of  several  layers  of  tissue,  and  thrown  an- 
teriorly, just  behind  the  sclero-corneal  junction, 
into  numerous  longitudinal  folds,  called  the  ciliary 
processes.  An  extension  from  the  ciliary  processes 
passes  forward,  constituting  the  iris,  which  is  a 
perforated  vascular  connective-tissue  and  muscular 
curtain,  suspended  behind  the  cornea,  and  con- 
nected peripherally,  near  the  sclero-corneal  junction, 
with  a  connective-tissue  structure  called  the  liga- 
mentum  pectinatum. 

Passing  backward  from   the  ligamentum  pectina- 

232 


THE  EYE.  233 

turn,  between   the  ciliary  processes  and  the   sclera, 
and  attached  posteriorly  to  the  choroid,  is  a  muscle 
having  the  form  of  a  flattened  ring,  thickest  in  front, 
called    the    ciliary    muscle.      The    direction    of    the 
muscle-cells  in   the   ciliary  muscle,  which  are  of  the 
smooth  variety,  is  in  part  meridional  or  oblique,  in 
part    circular.     The    ciliary    processes    and    muscle 
form  together  the  greater  part  of  a  structure  known 
as  the  ciliary  body.     The  retina,  the   innermost   of 
the  layers  forming  the  wall  of   the  eyeball,  spreads 
out  from  the  point  of  entrance   of   the  optic  nerve 
over  the  inner  surface  of  the  choroid.     At  about  a 
third  of   the  distance   back   from   the   front   of   the 
eye,  the   nerve-elements  of  the    retina   cease    in    a 
wavy  line,  called   the  ora  serrata ;    certain  cellular 
elements   continue,   however,   over  the   ciliary   pro- 
cesses, under  the  name  of  pars  ciliaris  retince. 

The  crystalline  lens  is  suspended  close  behind  the 
iris  by  a  firm,  delicate,  fibrillated  membrane,  called 
the  suspensory  ligament,  which  is  attached,  on  the 
one  hand,  to  a  membrane  covering  the  ciliary  pro- 
cesses, and  on  the  other  to  the  capsule  of  the  lens. 

The  cavity  of  the  eyeball  is  divided  by  the  lens 
and  its  suspensory  ligament  into  two  chambers,* 
the  anterior  and  smaller  of  which  is  filled  with  a 
homogeneous  fluid   the  aqueous  fluid ;  the  posterior, 

*  By  the  anterior  and  posterior  chambers  of  the  eye,  ophthalmolo- 
gists at  present  mean  the  cavities  in  front  of  the  lens,  separated  by 
the  iris,  and  formerly  regarded  as  constituting  the  anterior  chamber 
alone,  while  that  containing  the  vitreous  was  called  the  posterior. 


234  NORMAL  HISTOLOGY. 

with  a  gelatinous  substance,  the  vitreous  body,  which 
presents  an  ill-defined  lamellar  structure,  and  some- 
times contains  a  variable  number  of  ill-defined 
more  or  less  granular  cells.  The  vitreous  is  sur- 
rounded by  a  delicate  membrane,  called  the  hyaloid 
membrane,  which  is  closely  connected  posteriorly 
with  the  lining  membrane  of  the  retina,  and  is 
hardly  to  be  differentiated  from  it.  The  hyaloid 
membrane  is  thickened  and  fibrillated  over  the 
ciliary  processes,  where  it  is  called  the  zonula  ciliaris, 
and  a  prolongation  forward  from  this  constitutes 
the  suspensory  ligament  of  the  lens. 

Having  thus  briefly  described  the  general  struc- 
ture of  the  eye,  it  remains  for  us  to  consider  some 
of  its  parts  somewhat  more  in  detail ;  the  scope  of 
this  manual  will  not  permit  us,  however,  to  make 
an  extended  study  of  all  or  even  any  of  the  struc- 
tures in  the  eye ;  we  shall  be  obliged  to  confine 
ourselves  to  the  more  marked  structural  features  of 
the  cornea  and  sclera,  the  posterior  portions  of  the 
choroid  and  retina,  the  iris  and  crystalline  lens. 

THE    SCLERA. 

The  sclera  is  composed  of  very  closely  interwoven 
connective-tissue  fibres,  with  fine  elastic  fibres,  the 
latter  most  abundant  near  the  inner  surface.  Be- 
tween the  fibres,  which  have  little  regularity  in  their 
arrangement,  lie  flat  connective-tissue  cells,  a  certain 
number  of  which  frequently  contain  pigment-gran- 


THE  EYE.  235 

ules.  On  the  external  surface  the  sclera  sends  off 
delicate  fibres  anteriorly  into  the  subconjunctival 
tissue,  while  posteriori}-,  behind  the  muscle  tendons, 
they  join  to  form  the  wall  of  a  lymph-sac,  called  the 
capsule  of  Tenon.  On  the  inner  surface  certain 
fibres  pass  directly  over  into  the  choroid  ;  others 
form  the  outer  wall  of  a  lymph-sac  between  the 
sclera  and  choroid,  and  called,  on  account  of  its 
yellow  color,  the  lamina  fusca;  it  resembles  in 
structure  the  outer  layers  of  the  choroid,  presently 
to  be  described  as  the  membrana  supra-choroidea,  of 
which  it  is  indeed  a  part.  In  man  and  many  animals, 
the  opening  in  the  posterior  segment  of  the  sclera, 
through  which  the  optic  nerve  passes,  is  crossed 
by  a  net-work  of  connective-tissue  fibres  ;  these  pass 
in  from  the  sclera  on  all  sides,  and  surround  the 
delicate  bundles  of  nerve-fibres  of  the  opticus,  form- 
ing the  lamina  cribrosa. 

THE    CORNEA. 

The  cornea  is  directly  continuous  at  its  periph- 
ery with  the  sclera,  but  differs  from  it  in  struc- 
ture in  many  particulars,  among  the  more  promi- 
nent of  which  are,  the  more  regular  lamellar 
arrangement  of  its  connective-tissue  basement-sub- 
stance, the  greater  transparency  of  the  latter,  the 
peculiar  form  of  its  cellular  elements,  and  the  free 
surfaces  covered  with  cells. 

In  a  thin  section  of  the   cornea,  perpendicular  to 


236  NORMAL  HISTOLOGY. 

its  surface,  we  recognize,  passing  from  before  back- 
ward, five  layers:  I.  A  stratified  layer  of  epithelial 
cells — the  ajitcrior  corneal  epithelium — consisting  of 
cells  resembling  in  general  form  and  arrangement 
those  of  the  epidermis;  that  is,  we  have  in  the 
deepest  layer,  cylindrical  cells,  passing  over  into 
polyhedral,  and  these  into  flattened  cells  at  the 
surface  ;  2.  The  anterior  epithelium  rests  on  a 
dense  transparent  membrane,  called  the  anterior 
basal  membrane,  ox  lamina  elastica  anterior,  which 
is  composed  of  closely  packed  fibrillar  ;  3.  The  body 
of  the  cornea — substantia  propria  cornea — is  com- 
posed of  connective  tissue  whose  characteristics  we 
have  already  studied  ;  4.  Lying  closely  upon  the 
posterior  surface  of  the  last  layer,  is  a  thin,  appar- 
ently structureless  membrane — the  membrane  of 
Descemet  or  lamina  elastica  posterior — upon  which 
lies  :  5.  A  single  layer  of  flattened  polyhedral  cells, 
called  the  endothelium  of  Descemet. 

Except  at  its  extreme  periphery,  the  cornea 
contains  no  blood-vessels.  Nerves,  on  the  other 
hand,  are  very  abundant.  These,  in  larger  and 
smaller  trunks,  enter  the  cornea  at  the  periph- 
ery, and  dividing  and  subdividing,  break  up  into 
bundles  of  extremely  delicate  fibrils,  some  of 
which  are  distributed  to  the  superficial,  others  to 
the  deep,  layers  of  the  cornea.  These  fibrils  form 
extraordinarily  delicate  and  intricate  plexuses,  and 
are  finally   resolved   into  the   ultimate  nerve-fibrils 


THE  EYE.  237 

which,  often  finely  beaded,  pass  off  to  their  termi- 
nations. The  exact  mode  of  termination  of  the 
nerve-fibrils  in  the*  cornea  is  not  vet  sufficiently 
definitely  known  ;  certain  of  them,  however,  seem 
to  pass  between  the  anterior  epithelial  cells,  and  are 
believed  by  some  investigators  to  end  in  free  ex- 
tremities at  the  surface. 

THE    CHOROID. 

In  the  posterior  portion  of  the  eye,  the  choroid 
presents  four  layers,  which,  although  intimately 
connected,  and  presenting  no  sharp  line  of  division, 
may  yet  be  more  or  less  completely  separated  by  a 
careful  dissection.  Directly  beneath  the  lamina 
fusca  of  the  sclera,  and  forming  the  inner  wall  of 
the  above-mentioned  lymph-sac,  is  the  outermost 
layer,  called  the  lamina  supra-choroidca ;  it  is  com- 
posed of  a  series  of  superimposed  connective-tissue 
membranes  containing  delicate  elastic  fibres  and 
numerous  flattened,  irregular-shaped,  often  branch- 
ing pigmented  cells.  The  layers  are  covered  with 
endothelium,  and  the  spaces  between  them  are 
lymph-spaces  or  sinuses. 

Within  the  lamina  suprachoroidea  lies  a  layer, 
containing  the  larger  arteries  and  veins  of  the 
choroid,  called  the  external  vascular  layer,  or  the 
layer  of  Haller.  This  layer  is  composed  of  a 
groundwork  similar  to  the  suprachoroidea,  in  which 
the  vessels  are  embedded,  the   arteries  being  "often 


238  NORMAL  HISTOLOGY. 

closely  surrounded  by   dense  masses   of  pigmented 
cells. 

The  inner  vascular  layer,  called  the  chorio-capil- 
laris,  follows  next,  and  consists  almost  entirely  of  a 
very  dense  net-work  of  broad  capillary  blood-ves- 
sels. Finally,  the  choroid  is  limited  within,  by  an 
extremely  delicate,  finely  striated  membrane,  called 
the  lamina  vitrea,  or  membrane  of  Bruch. 

THE    IRIS. 

The  iris  is  a  thin  connective-tissue  membrane, 
pierced  near  the  centre  by  an  opening,  the  pupil, 
and  joined  at  the  periphery  to  the  ligamentum 
pectinatum  and  the  ciliary  body.  The  bulk  of  the 
iris,  the  substantia  propria,  consists  of  delicate  inter- 
lacing connective-tissue  fibres,  among  which  arc 
numerous  variously  shaped,  often  branching  pig- 
mented and  unpigmented  cells. 

It  contains  numerous  blood-vessels  which  are 
characterized  by  an  extraordinary  thickness  of  the 
walls.  Near  the  pupillary  margin  lies  a  circular 
band  of  smooth  muscle-cells — spJiinctcr  pupillce — 
while  radiating  bands  of  similar  cells  passing  from 
the  periphery  toward  the  pupil — dilator  pupillce — 
are  found  in  certain  animals,  but  not  in  man.  The 
anterior  surface  is  covered  by  a  layer  of  endothe- 
lial cells,  while  the  posterior  surface  is  covered  by 
an  irregular  thicker  layer  of  polyhedral  cells,  which 
are  densely  crowded  with  pigment,  and  constitute 
the  so-called  uvea 


THE  EYE.  259 


THE    RETINA. 


•  Of  all  the  animal  structures  the  retina  is  one  of 
the  most  delicate,  complicated,  and  difficult  of 
study,  and  we  can  do  little  more  here  than  indicate 
briefly  the  general  grouping  of  its  elements.  It 
consists  of  a  connective-tissue  framework  by  which 
the  nerve-elements  are  supported,  and  with  which 
they  are  most  intimately  associated  ;  and  in  some 
cases  it  is  as  yet  impossible  to  say  to  which  variety 
of  tissue  a  given  element  belongs.  We  distinguish, 
in  typical  parts,  ten  layers  ;  commencing  from 
within,  they  may  be  enumerated  as  follows  : 

1.  Membrana  limitans  interna. 

2.  Layer  of  nerve-fibres. 

3.  Layer  of  ganglion-cells, 

4.  Internal  molecular  layer. 

5.  Internal  nuclear  layer, 

6.  External  molecular  layer. 

7.  External  nuclear  layer. 

8.  Membrana  limitans  externa. 

9.  Layer  of  rods  and  cones. 
10.   Pigment  layer. 

The  limiting  membranes,  the  outer  of  which  is 
perforated  by  numerous  openings,  are  very  delicate 
and  homogeneous,  and  belong  to  the  connective- 
tissue  framework.  In  the  layer  of  nerve-fibres, 
which  is  thickest  around  the  entrance  of  the  optic 
nerve, — thus  forming  the  papilla,— the  fibres  spread 


240  NORMAL  HISTOLOGY. 

out,  intricately  interlacing,  into  a  thin  sheet,  and 
then  pass  outward  into  the  next  layer  to  join  the 
ganglion-cells.  These,  which  have  the  general 
characters  of  branching  nerve-cells,  send  numerous 
processes  into  the  internal  molecular  layer,  where 
they  break  up  into  an  extremely  delicate  fibrillar 
net-work  associated  with  the  connective-tissue 
framework.  Most  of  the  nuclei  in  the  internal  nu- 
clear layer  are  believed  to  belong  to  small  nerve- 
cells,  while  the  larger  ones  belong  to  the  connective- 
tissue  framework.  In  the  external  molecular  layer, 
again,  we  have  a  delicate  net-work  of  nerve-fibrils 
intermingled  with  connective  tissue.  The  nuclei  of 
the  external  nuclear  layer  seem  to  belong  exclusively 
to  nerve-elements,  and  are  directly  connected,  by 
processes  which  pass  through  the  openings  in  the 
external  limiting  membrane,  with  the  rods  and 
cones. 

Of  the  rods  and  cones,  which  within  the  limits  of 
this  book  cannot  even  in  a  general  way  be  ade- 
quately described,  the  rods  are  the  longer,  are  usu- 
ally somewhat  pointed  at  the  inner  extremity  where 
they  join  the  nerve-elements  of  the  outer  nuclear 
layer  ;  the  cones  are  shorter,  are  connected  also  with 
nerve-elements  within,  and  terminate  externally  in 
pointed  or  rounded  extremities. 

The  connective-tissue  elements  of  the  retina  con- 
sist, in  certain  layers,  of  broad,  irregular  radial  fibres 
forming  frequent   inosculations,  and,  in  the  molecu- 


THE  EYE.  241 

lar  layers,  of  a  delicate  reticulum,  within  which  the 
nerve  fibrils  ramify. 

The  pigmented  epithelium  of  the  retina  consists 
of  large  polyhedral  cells  set  together  side  by  side, 
and  forming  a  continuous  layer  over  the  distal  ends 
of  the  rods.  As  seen  from  the  side,  as  we  have  al- 
ready studied  them,  p.  24,  they  appear  like  flat 
pentagonal  or  hexagonal  plates,  but  when  seen  in 
profile  while  in  situ,  it  is  evident  that  they  send 
down  long,  slender  pigmented  processes  between 
the  rods. 

The  outer  portion  of  these  cells,  that  next  to  the 
choroid,  usually  contains  the  nucleus  and  but  little 
pigment,  while  the  amount  of  pigment  in  the  pro- 
cesses seems  to  depend  upon  whether  the  eye  had 
been  exposed  to  light  or  not  immediately  before 
death.  For  it  has  been  recently  shown  that  in  some 
animals  the  pigment  particles  under  the  influence  of 
light  can  move  within  the  narrow  cell-processes  so 
as  to  be  now  collected  within  the  inner  portion  of 
the  cell-body,  and  again  grouped  in  larger  and 
smaller  masses  betwen  the  rods.  In  this  movement 
the  pigment  particles  are  themselves  passive,  the 
change  in  position  being  due  to  active  movements 
in  the  protoplasm,  induced  by  light. 

The  larger  arteries  and  veins  ramify  beneath  the 
internal  limiting  membrane  in  the  layer  of  nerve- 
fibres,  and  from  these  blood  is  distributed  outward 
to  all  the  layers,  as  far  as  to  the  external  nuclear 
layer,  beyond  which  no  blood-vessels  are  found. 


242  NORMAL  HISTOLOGY. 

THE    LEX  3. 

The  lens  is  a  transparent  double  convex  body,  of 
sufficient  firmness  to  maintain  its  form  when  re- 
moved from  the  eye,  and  is  enclosed  in  a  homoge- 
neous elastic  capsule  which  is  thicker  on  the  ante- 
rior than  on  the  posterior  surface.  To  the  periphe- 
ral zone  of  the  capsule,  on  both  anterior  and 
posterior  surfaces,  the  suspensory  ligament  is  firmly 
attached.  The  body  of  the  lens,  although  perfectly 
transparent,  is  by  no  means  structureless.  Behind 
the  anterior  wall  of  the  capsule  lies  a  single  layer  of 
flattened  polygonal  cells,  which  at  the  equator 
gradually  lengthen  out  to  form  very  much  elongated, 
band-like  nucleated  cells,  or  lens-fibres,  which  run- 
ning meridionally,  and  joined  by  interfibrillar  ce- 
ment-substance make  up  the  greater  part  of  the 
body  of  the  lens.  The  lens-fibres  are  slightly  ridged 
upon  the  surface,  and  have,  on  transverse  section,  a 
flattened  hexagonal  form  ;  they  are  so  intimately 
joined  to  one  another  at  their  sides  by  the  cement- 
substance,  that  under  certain  circumstances  they 
may  be  peeled  off  in  layers. 

The  course  of  the  fibres  in  the  lens  is  somewhat 
complicated,  but  it  may  be  said  in  general  that  they 
run  meridionally  from  one-half  of  the  lens,  in  broad 
sweeps,  over  into  the  other  ;  inasmuch,  however,  as 
the  individual  fibres  are  not  long  enough  to  reach 
the  entire  distance  from  one  pole  of  the  lens  around 


THE  E  YE.  243 

to  the  other,  they  commence  along  certain  definite 
lines  at  varying  distances  from  the  poles ;  and  the 
farther  from  one  pole  one  end  of  the  fibre  is,  the 
nearer  to  the  other  will  its  other  end  lie.  In  the 
young  human  lens,  the  lines  from  which  the  fibres 
start  may  be  seen  on  the  anterior  and  posterior  sur- 
faces, under  certain  circumstances,  in  the  form  of  a 
three-rayed  star;  in  the  adult,  the  rays  usually  part 
at  the  ends,  giving  rise  to  secondary  rays. 

THE    EYELIDS. 

These  are  formed  in  general  by  a  plate  of  connec- 
tive tissue,  which  toward  the  free  border  is  very 
dense  and  firm,  and  called  the  tarsal  cartilage,  or  tar- 
sus ;  they  are  covered  on  the  outside  by  skin,  on  the 
inside  by  the  conjunctival  mucous  membrane ; 
between  the  tarsus  and  the  skin  lie  the  bun- 
dles of  the  musculus  orbicularis.  The  tarsus, 
which  is,  in  no  sense,  cartilage,  is  a  plate 
of  very  dense  and  firm  fibrillar  connective  tissue, 
containing  ordinary  flattened  connective-tissue  cells, 
and  is  closely  connected  within  with  the  palpebral 
conjunctiva.  Embedded  within  the  tarsus  lie  the 
Meibomian  glands,  thirty  to  forty  in  number  in  each 
lid.  They  consist  of  numerous  vesicular  alveoli, 
lined  with  short  cylindrical  cells,  arranged  along  and 
opening  into  long  excretory  ducts,  which  are  lined 
with  laminated  epithelium,  and  open  on  the  inner 
border   of   the   edge    of  the   lids.     They  are   some- 


244  NORMAL  HISTOLOGY. 

what  modified  sebaceous  glands,  but,  unlike  most 
sebaceous  glands  are  not  connected  with  hair-fol- 
licles. 

The  skin  of  the  eyelids  is  somewhat  thinner  than 
that  of  the  face,  is  beset  with  delicate  hairs,  and 
supplied  with  sweat-glands  and  sebaceous  follicles. 
It  passes  over  on  to  the  edge  of  the  lids,  at  the  in- 
ner border  of  which  it  becomes  continuous  with  the 
mucous  membrane.  The  eyelashes  are  inserted 
obliquely  into  the  edge  of  the  lid,  in  from  two  to 
four  rows  ;  and  the  follicles,  which  are  quite  deep, 
are  furnished  with  sebaceous  glands. 

The  conjunctival  mucous  membrane  of  the  lids 
consists  of  a  basis  substance  of  loose  fibrillar  connec- 
tive tissue  containing  a  few  elastic  fibres  and  numer- 
ous small  spheroidal  and  branching  cells.  The 
epithelium  is  laminated,  consisting  of  a  deep  layer  of 
polyhedral  cells,  then  more  superficially  of  more  or 
less  columnar  cells. 

The  epithelium  of  the  bulbar  conjunctiva  ap- 
proaches more  and  more  closely  in  structure  that  of 
the  cornea,  as  we  pass  from  the  lid  over  toward  the 
sclero-corneal  junction. 

Small  racemose  glands,  called  accessory  tcar-glaiids, 
are  often  seen  opening  on  the  surface  of  the  mucous 
membrane.  In  addition  to  the  striated  muscular 
bundles  of  the  orbicularis,  smooth  muscle-cells, 
forming  a  membranous  layer,  occur  beneath  the  con- 
junctiva on  the  orbital  portion  of  the  lids. 


THE  EYE.  245 

PRACTICAL   STUDY. 

General  Dissection  of  the  Eye. — For  this  purpose  a 
large  eye,  like  that  of  the  sheep  or  ox,  is  preferable  ;  it 
should  be  opened  by  a  short  incision  through  the  scle- 
rotic, so  that  the  fluid  can  readily  come  into  contact  with 
the  parts  within,  and  placed  in  Miiller's  fluid  ;  after  two 
weeks  it  is  carefully  washed  and  placed  in  alcohol  for  a 
week,  when  the  dissection  may  be  made. 

The  eye  should  be  divided  with  a  sharp  razor  into 
lateral  halves,  the  section  passing  through  the  optic 
nerve.  The  cut  surface  shows  clearly  the  general  rela- 
tions of  the  parts  ;  cornea,  iris,  lens,  ciliary  body,  vitreous, 
retina,  choroid,  and  sclera  are  seen  in  approximately  nor- 
mal relations  to  one  another. 

If  the  vitreous  be  now  removed  from  one  of  the  halves, 
the  retina  and  ciliary  body  come  more  fully  into  view. 
As  the  zonula  ciliaris  approaches  the  edge  of  the  lens, 
it  divides  into  two  layers,  which  pass,  one  to  the  anterior, 
the  other  to  the  posterior  surface  of  the  body,  forming 
the  suspensory  ligament  ;  the  slit-like  opening  between 
the  layers  is  called  the  canal  of  Petit,  which  may  be  seen 
by  pulling  the  lens  slightly  backward,  when  the  layers  will 
separate. 

Now  seizing  the  half  of  the  lens  with  forceps  and  care- 
fully drawing  it  downward  and  backward  away  from  the 
iris,  the  zonula  ciliaris,  in  the  form  of  a  folded  fringe- 
like membrane,  will  be  separated  from  the  surface  of  the 
ciliary  body.  A  portion  of  this,  in  connection  with  a 
fragment  of  the  lens-capsule,  is  detached  from  the  lens, 
stained  deeply  in  eosin,  and  mounted  in  glycerin.    After 


246  NORMAL  HISTOLOGY. 

the  removal  of  the  lens,  the  form  and  attachment  of 
the  iris  are  readily  seen. 

In  the  same  half  of  the  eye,  the  layers  of  the  choroid 
may  be  demonstrated.  For  this  purpose  the  retina  is 
pulled  off,  and  the  pigmented  cells,  which  are  apt  to  ad- 
here to  the  inner  surface  of  the  choroid,  are  brushed  or 
scraped  off.  The  eye  is  now  firmly  pinned  with  the  flat 
side  down  to  a  flat  bit  of  cork  which  has  been  attached 
to  a  piece  of  sheet-lead  and  sunk  in  a  dish  of  water  deep 
enough  to  cover  the  eve.  A  window  about  a  centimetre 
square  is  now  cut  through  the  sclera,  including  the  sclero- 
corneal  junction,  and  the  bit  of  sclera  having  been  re- 
moved, care  being  taken  not  to  disturb  the  ciliary  body, 
the  membrana  suprachoroidea  will  be  seen  as  a  brown 
loose  tissue  floating  at  the  surface  of  the  choroid.  Bits 
of  this  are  pulled  off  with  the  forceps,  stained  with  he- 
matoxylin, floated  smoothly  on  to  a  slide  immersed  in 
water,  and  mounted  in  glycerin.  The  floating  shreds 
of  the  suprachoroidea  which  remain  after  suitable  speci- 
mens have  been  obtained,  should  now  be  pulled  from  the 
eye,  when  the  vascular  layer  of  Haller  will  come  into 
view. 

The  separation  of  the  three  remaining  layers  is  not 
easy  under  the  most  favorable  conditions,  and  is  espe- 
cially difficult  in  the  eye  of  the  ox  and  sheep,  where  the 
layers  are  rendered  more  complicated  and  difficult  of 
separation  by  the  presence  of  a  mass  of  interlacing 
fibres.  Haller's  layer,  however,  and  the  membrana 
chorio-capillaris  may  be,  with  care,  stripped  off  in  pieces 
sufficiently  thin  for  demonstration  ;  sometimes  a  frag- 
ment   will  be   obtained,  which,   especially  at   the   edges. 


THE  E  YE.  247 

will  show  both  of  the  vascular  layers  at  once.     They  are 
stained  double  and  mounted  in  glycerin  or  balsam. 

Cornea  and  Sclera. — A  fresh  eye  from  the  rabbit  or  dog 
is  hardened  in  Miiller's  fluid  and  alcohol.  The  cornea 
should  now  be  excised  close  to  the  sclero-corneal  junc- 
tion, embedded  in  wax  or  liver,  and  thin  sections  made 
perpendicular  to  the  surface,  including  the  entire  thick- 
ness. They  may  be  stained  double  and  mounted  in 
glycerin. 

A  transverse  section  may  be  made  from  a  bit  of  the 
sclera  from  the  same  eye  ;  stained  double  and  mounted 
in  balsam.  4 

Sclero-comeal  Junction,  and  Iris. — The  cornea  and 
sclera,  in  their  relations  to  one  another,  and  the  ciliary 
bodies,  and  iris,  may  be  examined  in  a  specimen  pre- 
pared as  follows  :  An  eye — that  of  the  pig  is  best,  if  a 
fresh  human  eye  cannot  be  procured — is  placed,  after 
making  a  short  incision  in  the  sclera,  in  Miiller's  fluid, 
where  it  remains  for  two  weeks,  the  fluid  being  changed 
once  or  twice  ;  it  is  then  soaked  for  an  hour  or  two  in 
water,  and  the  hardening  completed  in  dilute  and  strong 
alcohol.  A  bit  is  excised,  including  the  sclero-corneal 
junction  and  adjacent  parts,  and  embedded  in  cacao 
butter  by  the  following  method. 

This  cacao-butter  method  of  embedding  is  of  great 
use  in  making  sections  of  delicate  parts  where  the  tis- 
sues enclose  spaces,  or  are  not  close  enough  together  or 
firm  enough  to  afford  a  sufficient  resistance  to  the  razor. 
The  piece  of  tissue  is  transferred  from  alcohol  to  a  small 
dish  of  oil  of  cloves,  which  in  a  short  time  will  pene- 
trate  the  tissue  and  render  it   transparent.     It   is  then 


248  NORMAL  HISTOLOGY. 

placed  in  melted  cacao  butter,  whose  temperature  is 
kept  just  above  its  melting  point,  on  a  water-bath,  for  a 
couple  of  hours.  It  is  now  placed  in  a  little  box  made 
from  writing  paper,  in  such  a  position  that  sections  can  be 
made  in  a  suitable  direction  at  the  proper  time,  and  the 
box  filled  with  the  melted  cacao  butter  or  with  the  wax 
mixture  described  on  page  II  When  the  wax  is  cold 
the  whole  forms  a  solid  mass  of  suitable  consistence  for 
cutting,  and  the  wax  serves  to  hold  the  delicate  parts 
together  while  the  section  is  being  made.  Sections  are 
made  at  right  angles  to  the  ciliary  body. 

The  wax  and  the  cacao  butter  must  now  be  dissolved 
out  by  oil  of  cloves,  the  latter  replaced  by  alcohol,  and 
this  by  water  ;  the  sections  are  then  stained  double  and 
mounted  in  balsam.  This  method  seems  Ions;  and  com- 
plicated,  but  the  excellence  of  the  sections  of  delicate 
parts  obtained  by  its  use  will  well  repay  the  worker  for 
his  pains. 

Lens  Fibres  by  Teasing. — A  short  incision  is  made 
through  the  sclera  of  a  fresh  eye  which  is  soaked  for 
three  or  four  days  in  a  mixture  of  alcohol  and  water,  1 
to  2.  The  lens  will  be  found  on  removal  to  be  white 
and  soft,  and  readily  breaks  up  into  layers  ;  a  bit  of  one 
of  these  is  teased  on  a  slide  in  eosin-glycerin  and 
mounted  in  the  same. 

Sections  of  the  Lens. — The  eye  of  a  rabbit  or  pig 
should  be  kept  for  a  fortnight  in  Muller's  fluid  ;  the 
lens  is  then  removed,  care  being  taken  not  to  rupture 
the  capsule,  and  placed  for  a  day  or  two  in  dilute  and 
then  in  strong  alcohol.  It  is  embedded  in  wax,  and  thin 
sections,  made  in  an  antero-posterior  direction  through 


77 IE  E  YE. 


249 


the  centre,  are  stained  double   and  mounted   in   glycerin 
or  balsam. 

Transverse  Sections  of  the  Retina. — In  a  perfectly  fresh 
human  or  pig's  eye,  one  or  two  small  openings  are  made 
through  the  sclera  and  it  is  placed  in  Muller's  fluid  ; 
after  a  week  the  eye  may  be  cut  across  just  behind  the 
sclero-corneal  junction,  and  the  posterior  segment  placed 
in  fresh  Muller's  fluid.  After  another  week  it  is  trans- 
ferred for  twenty-four  hours  to  dilute,  and  then  put  for 
two  or  three  days  in  strong  alcohol.  A  small  piece  is 
now  cut  from  the  retina  at  a  little  distance  from  the 
optic-nerve  entrance  and  embedded  in  cacao  butter  (see 
above).  Thin  transverse  sections  are  made,  the  cacao 
butter  dissolved  out  in  the  usual  way,  and  stained  double 
and  mounted  in  balsam.  By  this  method  the  general 
arrangement  of  the  layers  is  well  shown,  but  many  of 
the  finer  details  of  structure  are  obscure. 


INDEX 


Acetic  Acid,  action  of,  on  tissues 

Air-vesicles  oi  lungs,  demonstration  of  epithelium  of 

structure  of 
Albuginea,  of  ovary 

of  testicle 
Alcohol,  as  a  hardening  and  preservative  agent 
Alveolar  passages  of  lungs,  structure  of 
Ammonium  bichr.,  as  a  hardening  agent 
Ammonium  chromate  (neutral),  as  a  hardening  agent 
Amoeboid  movement,  in  white  blood-cells,  character  of 

method  of  studj'ing 
Aniline-red,  as  a  staining  agent    . 
Arachnoid,  of  brain 
Areolar  tissue 
Arteries,  structure  of 

method  of  studying 
Arterioles       .... 
Asphalt  varnish,  for  enclosing  specimens 
Axis  cylinder  of  nerve-fibres 

Balsam,  method  of  mounting  in  . 

Blood 

coagulation  of 
free  granules  in 
method  of  studying  fresh     . 
plasma  of 
Blood-cells,  red 

action  of  water  on 

change  of  form  in 

coloring  matter  of 

difference  in,  in  different  animals 

nucleated,  in  marrow  of  bone 

in  spleen 
number  of 

251 


PAGE 

16 

176 

173 
198 
188 

3 

172 

4 

4 
81 
89 

35 
218 

34 

ISO,  121 
I26 
I20 

24 
IO4 

IS 

80 

85 
84 
87 
85 

82 
82 

83 

84 
84 
67 

139 

83 


252 


INDEX. 


jmooa-cens,  rea 

PAGE 

number  of,  relative  to  white  blood-cells       .            .                   83 

origin  of    . 

86 

size  of 

. 

83 

stroma  of  . 

. 

84 

varieties  in  - 

84 

white     . 

80 

amceboid  movements  in 

8l,  89 

demonstration  of  nuclei  in 

83 

origin  of    . 

86 

Blood-crystals           ..... 

84,88 

Blood-vessels,  adventitia  of 

120 

arteries        .           .           . 

120,  121 

capillaries    . 

119 

classification  of 

rig 

intima  of 

T20 

musculosa  of 

.                         120 

method  of  preparing,  for  study 

I25,  126 

veins 

121 

Blue  gelatine  mixture  for  injecting,  formula  foi 

prep. 

of 

IO 

Bone    ...... 

6l 

canaliculi  of     . 

62 

cells       ..... 

62 

decalcified,  method  of  preparing   . 

67 

developing,  preparation  of  . 

77 

development  of,  intra-cartilaginous 

7i 

sub-periosteal    \  . 

74 

intra-membranous 

75 

hard,  method  of  preparing  sections  of 

69 

Haversian  canals  of  . 

64 

marrow  of 

65 

periosteum  of 

65 

Bone-tissue,  spongy 

62 

compact 

63 

Sharpey's  fibres  in 

64,  75 

Brain,  arachnoid  of  . 

218 

blood-vessels  of 

•                  217 

dura  mater 

t                  217 

general  structure  of 

215 

pia  mater 

218 

preparation  of  sections  of 

•                  219 

structure  of  cortex  of  cerebellum 

217 

structure  of  cortex  of  cerebrum    . 

.                  216 

structure  of  white  matter  ( 

>f 

215 

INDEX. 


253 


Bronchi,  large,  structure  of 

small,  structure  of  . 

Bruch,  membrane  of,  in  choroid  . 
Brunner,  glands  of,  in  duodenum 
Budding,  cell  multiplication  by     . 


Cacao  Butter,  method  of  embedding  in 
Canada  balsam,  hard,  for  mounting  bone 

solution  in  chloroform  for  mounting 
Canaliculi  of  bone    .... 
Capillaries,  structure  of 

method  of  studying     . 
Capsule,  of  cartilage  cells  . 

supra-renal 

Carmine,  preparation  and  use  of,  as  a  staining  agent 
Cartilage        ..... 
cells  of 

artificial  shrinkage  of  . 
classification  of    . 
f£>i-o-     .... 

preparation  of     . 
fibro-elastic 

preparation  of    . 
hyaline,  basement  substance  of 
preparation  of    . 
Cells,  body  of  ... 

bone     ..... 
ciliated,  from  frog's  mouth 
classification  of 
connective-tissue 
corneal,  seen  on  edge 
seen  on  flat 

relation  of,  to  basement  substance 
division  of 

endothelial     .... 
epithelial,  of  small  intestine 
ganglion  .... 

general  structure  of 
goblet,  in  small  intestine     . 
liver     ..... 
marrow  .... 

membrane  of  .  .  . 

nuclei  of        .  *  . 


PAGE 

170 
171 
238 
150 


247 

70 

14 

62 

119 

125 

56 

164 

7 

56 

56 

60 

57 
59 
59 
59 
61 

57 
61 

17 
62 

25 
22 

32 
40 

4i 
42 
21 

33 
149 
in 

17 
150 
157 

66 

19 
18 


254 


INDEX. 


PAGE 

Cells,  nucleoli  of      .......           ,                  19 

peptic  ..... 

147 

pigmented,  in  choroid 

36 

physiology  of 

19 

plasma             .... 

33 

practical  study  of 

23 

reproduction  of 

20 

tendon             .... 

36,38 

Cell-spaces  of  connective  tissue   . 

44 

Cement,  in  roots  of  teeth     . 

78 

Central  nervous  system 

.                   211 

Central  tendon,  in  diaphragm  of  rabbit 

127 

Chondrin        ..... 

58 

Chromic  acid  as  a  hardening  agent 

4 

Chyle  vessels,  in  small  intestine 

*5* 

Ciliary  motion,  in  cells  from  frog's  mouth 

25 

Coloring  tissues  (see  staining) 

6 

Conheim's  method  of  gold  staining 

39 

Coni  vasculosi  of  testicle    . 

189 

Connective  tissue,  classification  of 

27 

distribution  of,  in  body 

27 

fibrillar 

29 

of  nerves 

106 

origin  of.  in  embryo 

28 

practical  study  of 

33 

reticular 

54 

method  of  preparin 

55 

Corium  of  skin          .... 

222 

Corpus  Highmori  of  testicle 

188 

luteum  in  ovary      .... 

201 

spongiosum  of  penis 

196 

preparation  of 

197 

Decalcification  of  bone    . 

67 

Delafield's  method  of  preparing  haeniatoxylin 

6 

Dentine           ..... 

78 

Double -staining        .... 

8 

in  Canada  balsam  mounting 

14 

Dura  mater,  structure  of    . 

217 

preparation  of 

•                 219 

Elastic  Fibres  in  connective  tissue 

3° 

in  ligamentum  nuchse     . 

34 

INDEX, 


255 


Elastic  granules  in  connective  tissue 
Embedding  in  cacao  butter 
in  gum 
in  liver 
in  wax 
Emigration  of  white  blood-cells  . 
Enamel  cuticle 

of  teeth 
Endocardium,  structure  of 

method  of  studying 
Endogenous  cell  reproduction 
Endothelial  cells 
Endothelium,  of  mesentery 

of  omentum 
Eosin,  mounting  specimens  stained  with,  in  glycerin 

preparation  and  use  of 
Epidermis       .... 
Epithelial  cells  from  rabbit's  bladder 
Epithelium,  renal 
Erectile  tissue,  nature  of 
Eye 

aqueous  fluid  of 
canal  of  Petit 
capsule  of  Tenon 
chambers  of 
choroid  . 
ciliary  body 
cornea,  structure  of 

preparation  of 
conjunctiva 
dissection  of 
general  structure  of 
hyaloid  membrane 
iris,  structure  of 

preparation  of 
lamina  fusca 
lens,  structure  of 

preparation  of 
ora  serrata 
retina,  structure  of 

preparation  of 
sclera,  structure  of 
sclero-corneal  junction,  preparation  of 


TAGE 

31 
247 

115 
11 


79 
79 
123 
127 
22 
33 
45 
46 

13 

8 

220 

23 
182 

J95 
232 

233 
245 
235 
233 
237 
233 
235 
247 
244 
245 
23a 
234 
238 
247 
235 
242 
248 
233 
239 
249 
234 
247 


256 


INDEX. 


Eyes,  uspensory  ligament 

uvea. 

vitreous  body 

zonula  ciliaris 
Eyelids  .... 

Fat  Cells,  structure  of 

development  of         . 
Fat  tissue       .... 
fresh,  preparation  of 
method  of  studying,  in  adult 
method  of  studying,  in  young  animal 
Fatty  infiltration 
Fibres,  elastic,  in  connective  tissue 

fibrillated,  in  connective  tissue 
nerve 

muscle  .  ,  . 

Fibrillce  in  tail  tendon  of  mouse    . 

primitive  muscle   . 
Fibrillar  connective  tissue 
Fibrin  from  blood,  characters  of 

method  of  preparing 
Fibro-cartilage,  structure  of 
Fibro-elastic  cartilage 
Fission,  cell  multiplication  by 
Fresh  tissues,  method  of  examining 
Fuchsine  (see  aniline  red) 

Ganglion  Cells 
Gastro-intestinal  canal 
Gelatin,  for  injecting 
Gemmation,  cell  multiplication  by 
Generative  organs,  female 
male    . 
Germ  epithelium  of  oxzxy 
Giant  cells,  in  marrow  of  bone 
Gianuzzi,  crescents  of,  in  submaxillary  gland 
Glands,  Brunner's    . 

general  characters  of 

lymphatic    . 

mammary    . 

Meibomian  .  . 

peptic 


PAGE 

233 
238 
234 
234 
243 

5° 
51 
50 
52 
53 
53 
52 
30 
30 
104 

95 
34 
96 
29 
85 
90 

59 

59 

21 

2 

35 

in 

143 
10 
22 

J97 
188 
202 
66 
156 
150 
144 
129 

2C8 

243 
147 


INDEX. 


257 


Glands,  prostate 

racemose 

sebaceous    . 

submaxillary 

sweat 

thyroid 

tubular 

vesicular 
Glisson's  capsule  cf  liver 
Glomeruli  of  kidney 
Glycerin  as  a  mounting  medium 
Goblet  cells    . 

Gold  chloride  as  a  staining  agent 
Graafian  follicles,  structure  of 

development  of 


Hematoxylin,  preparation  and  use  of 
Haemoglobin,  characters  of 

preparation  of  crystals  of 
Kair,  ceils  in  cuticula  of 

follicle,  structure  of  . 
general  structure  and  relations  of 
papillae  of 

preparation  of  sections  of    . 
shaft,  structure  of 
Ha  ler's  layer  of  choroid    . 
Hardening  agents     .  .  . 

Haversian  lamellae   .  , 

canals     .  .  . 

Heart,  endocardium 
muscle  of 
valves  of 
Henle,  sheath  of 
Hensen's  line  in  muscle-fibre 
Highmori,  corpus,  in  testicle 
Hyaline  cartilage,  structure  of 

method  of  studying 

Indifferent  Fluids,  use  of 
Infundibula  of  lungs,  structure  of 
Injection,  interstitial 

of  blood-vessels 
Intestine,  large,  structure  cf 


PAGE 
192 

M5 
227 

155 
223 
166 

*45 

146 

i59 

179,  182 

12 

150 

39 
199 
202 

6 

S4 

88 

230 

225 

224 

225 

230 

226 

237 

3 

63 

64 

123 

101 

124 

106 

97 
X88 

57 
59 


172 
6 


153 


258 


INDEX. 


Intestine,  large,  method  of  studying 
small, 

agminated  glands  of 
Brunner's  glands 
chyle  vessels  of 
epithelial  cells  of 
follicles  of  Lieberkuhn 
goblet  cells  of 
lymph-vessels  of 
muscularis  mucosae  of 
method  of  stud}'  of 
PejTer's  patches 
solitary  follicles  of 
villi  of 

Kidney,  capsule  of 

connective  tissue  of 

convoluted  tubules  of 

cortex  of    . 

cortical  pyramids  of 

epithelium  of 

general  structure  of 

glomeruli  of 

Henle's  loops  of 

injected,  preparation  of 

intercalated  tubules  of 

labyrinth  of 

medulla  of 

medullary  rays  of 

method  of  examining 

papilla  of 

parenchyma  of     . 

position  of  tubules  in 

preparation  of  isolated  tubules  of 

rabbit's,  preparation  of 

straight  tubules  of 

uninjected,  preparation  of 

uriniferous  tubules  of 
Krause,  line  of,  in  muscle-fibres 

Lactation,  changes  of  mammary  gland  during 
Lacunae,  of  bone     ..... 
Morgagni,  in  urethra     . 


PAGE 

*54 
149 
152 
150 

151 
149 
149 
150 
151 
151 
154 
152 
35i 
149 

178 

185 
181 
178 
179 
182 
178 
179,  182 
181 
187 
181 
179 
178 
178 
185 
178 
1  So 
183 
186 
185 
tSi 
186 
180 
97 

209 

62 

194 


INDEX. 


259 


Lamelloe,  of  bone 
Leucocytes    . 
Lieberkuhn,  follicles  of,  in  small  intestine 
Ligamentum  nuchae  . 

LLttre,  glands  of,  in  urethra 
Liver  .... 

blood-vessels  of 
cells  of  ... 

preparation  of 
connective  tissue  of 
gall-vessels  of 
general  structure  of 
Glisson's  capsule  in 
injection  of  blood-vessels  of 

gall-vessels  of 
lobules  of 

lymphatic  vessels  of 
lymphoid  tissue  in    . 
pig's,  preparation  of 
human,  preparation  of 
Lungs,  air  vesicles  of 

alveolar  passages  of 
alveoli  of 
blood-vessels  of    . 
infundibula  of 
injected  blood-vessels  of 
lobules  of     . 
method  of  examining 
pigment  in    . 

uninjected,  preparation  of 
Lymph 

follicles 
Lymphatic  capillaries 
glands    .. 
spaces 
vessels    . 
Lymphoid  tissue 

Malpighian  Bodies,  of  kidney 
of  spleen 
Malpighian  layer,  of  skin 
Mammary  gland,  structure  of 
preparation  of 


PAGE 

63 
80 
149 
34 
194 
157 
15S 
157 
161 
161 
160 
158 
159 
162 
163 

'59 
161 
161 
162 
762 
172 
272 
172 

*74 
172 
177 
171 
375 
175 
175 
86 

134 
125 
129 
44 
124 
134 

179 

137 
220 
208 
SIO 


260 


IXDEX. 


Marrow,  of  bone,  structure  of 

preparation  of 
Medullary  sheath,  of  nerve-fibres 
Meibomian  glands 
Meissner's  corpuscles 

Menstruation,  changes  of  uterine  mucous  membrane 
Mesentery,  endothelium  of 
Motor-end  plates     ..... 
Mounting,  in  glycerin         .... 

in  Canada  balsam 
Mucin,  in  embryonal  tissue 
Mucous  tissue  ..... 

in  umbilical  cord,  preparation  of 
Muller's  fluid,  formula  for 
Muscle-cells,  smooth  .... 

isolation  of 
preparation  of,  from  frog's  bladder 
sections  of,  from  intestine 
Muscle,  erector  pilee. 
Muscle-fibres,  general  structure  of 
Hensen'sline  in    . 
Krause's  line  in 
nuclei  of 
preparation  of  fresh 

by  osmic  acid 
sections  from  tongue 
sarcolemma  of 
sarcous  elements  of 
Muscle-fibrilloe,  primitive 
Muscle-tissue,  classification  of 
of  heart 

preparation  of    . 
smooth 
striated  voluntary  - 

Nail,  structure  of    . 

preparation  of  sections  of 
Nerves,  connective  tissue  of 

Henle's  sheath  of 

intra-fascicular  connective  tissue  of 

lamellar  sheath  of 

peri-fascicular  connective  tissue  of 

preparation  of  sections  of 


PAGE 

65 

70 

104 

243 

22  3 

205 
45 
99 
12 

14 
49 
43 

5° 
5 

92 
93 
94 
94 
227 

95 
97 
97 
98 

99 
100 
100 

98 

97 
96 

91 

101 

102 

92 

95 

223 
229 
106 
106 
10S 
107 
107 


INDEX. 


26l 


Nerve-cells,  classification  of 

preparation  of,  from  brain 

from  spinal  cord 
processes  of    . 
sympathetic,  structure  of 

preparation  of 
Nerve-fibres,  axis-cylinder  of 
classification  of 
fresh,  study  of 
incisures  of  Schmidt  in 
inter-annular  segments  of 
medullary  sheath  of 
medullated 
neurilemma  of 
non-medullated 
physiological  significance  of  structure 
preparation  of,  with  osmic  acid 
with  silver 

transverse  sections  of 
Ranvier's  crosses  on 
of  Remak      .... 
sympathetic,  preparation  of 
termination  of,  in  nerve-centres 
in  the  periphery 
Nerve-tissue,  classification  of 
Neuroglia,  character  of,  in  spinal  cord 
Nodes,  lymph 

Nucleated  red  blood-cells,  in  marrow 
in  spleen 

Odontoblasts 

Oil  of  cloves,  use  of,  in  Canada  balsam  mounting 

Omentum,  endothelium  of 

Optical  sections 

Osmic  acid,  action  of,  on  tissues 

Osteoblasts     . 

origin  of 
Ovary,  connective  tissue  of 

general  structure  of 

Graafian  follicles  of 

preparation  of 
Ovum,  structure  of  . 

Pacinian  Bodies,  in  skin 


of 


PAGE 
III 
Il8 
117 
III 
112 
Il8 
IO4 
IO3 

"3 

106 

105 
104 
104 
105 
no 

116 
114 

"5 
114 

"5 
no 
118 
108 
109 
103 
213 
129 
67 
139 

78 

«S 

46 

126 

5 

671  73 

76 
198 
197 
199 
204 
200 

228 


262 


INDEX. 


Panniculus  adiposus 
Papillae,  of  skin        .  . 

of  hair 
Periosteum    . 
Peyer's  patches 
Pia-mater,  of  brain 

preparation  of 
of  cord     . 
Pigmented  cells,  of  retina 
of  choroid 
Pleura,  pulmonary,  structure  of 
Potassium  bichromate 
Preservative  agents 
Prickle-cells,  in  epidermis 
Pritchard's  method  for  gold  staining 
Prostate  gland,  structure  of 

preparation  of 
Protoplasm    . 
Pulp,  of  spleen 
of  tooth 
cords,  in  spleen 
Purkinje's  cells,  structure  of 

position  of,  in  cerebellum 


PAGE 

223 

322 

225 

65 
152 

218 
219 

211 

24 

36 

175 

4 
3 

221 

39 
192 

193 
18 

139 

78 

139 

112 

217 


Ranvier,  crosses  of,  on  nerve-fibres 
Remak,  nerve-fibres  of 
Respiratory  apparatus 
Rolando,  substantia  gelatinosa  of 

Sarcolemma 

Sarcous  elements 

Schmidt,  incisures  of,  in  nerve-fibres 

Schwann,  sheath  of 

Sebaceous  glands 

Section  cutting 

Seminiferous  tubules 

Serous  membranes 

Shaking,  method  of,  in  preparing  tissues 

Sharpey,  fibres  of,  in  bone 

Silver  nitrate,  methods  of  staining  with 

Skin,  blood-vessels  of 

corium  of 

epidermis  of 


116 
no 

168 
214 

96 

97 
106 

105 
237 

9 

i33,  189 

44 

55 

64i  75 

42,45 

223 

222 

220 


INDEX. 


263 


Skin,  from  finger  tip,  preparation  of 
general  structure  of 
horny  layer  of 
injected,  preparation  of 
mucous  layer  of 
negro's 
nerves  of 
papillae  of 
prickle-cells  of 
rete  Malpighi 
stratum  lucidum 
Spermatoblasts 
Spermatozoa,  development  of 
movements  of 
preparation  of 
structure  of 
Spinal  cord,  arrangement  of  gray  matter 
blood-vessels  of 
central  canal  of 
columns  of 
connective  tissue  of 
cornua  of 

general  structure  of 
gray  commissures  of 
gray  matter  of,  structure  of 
neuroglia 

sections,  preparation  of 
substantia  gelatinosa  of  Rolando 
white  commissure 
white  matter,  structure  of 
Spleen,  cavernous  venis  of 

distribution  of  blood  in 
follicles  of 
general  structure  of 
preparation  of 
pulp  of 
Staining  agents 
Stellulae  Verheyenii 
Stomach,  blood-vessels  of 

general  structure  of 
lenticular  glands  of 
mucous  glands  of 
nerves  of 


PAGE 

230 
220 
220 

22Q 
220 
229 
228 
222 
221 
222 
221 

i  go 
190 
192 
191 
191 
211 
214 
212 
212 
213 
212 
211 
212 

2I3 
213 
219 
214 
213 
212 
140 
140 

137 
136 
141 

139 
6 

185 
148 
146 
148 

147 

148 


264 


INDEX. 


Stomach,   peptic  glands  of 

preparation  of  sections  of 
Subcutaneous  connective  tissue,  cells  of 

fibres  of 
Submaxillary  gland,  preparation  of 

relations  and  structure  of 
Suprarenal  capsule,  preparation  of 

structure  of 
Sweat-glands 
Sympathetic,  cells  of 
fibres  of 

Teeth.  .... 

Tendon,  cell  of ,  demonstration  of 

fi  brillce  of,  demonstration  of 
Testicle,  preparation  of 

structure  of 
Tlryroid  gland,  formation  of  colloid  in 
general  structure  of 
preparation  of 
Trachea,  blood-vessels  of 
mucous  glands  of 
preparation  of 
structure  of 


Umbilical  Cord,  mucous  tissue  in 
Urethra,  general  structure  of 

lacunse  Morgagni  of 
Littre's  glands  of 

preparation  of 
Uriniferous  tubules 
Uterus,  general  structure  of 

mucous  membrane  of 

preparation  of 

Vagina,  general  structure  of 
mucous  membrane  of 
preparation  of 
Vas  deferens 
Veins,  preparation  of 
structure  of  . 
valves  of 


PAGE 

i47 

35 
33 
157 
155 
164 

165 
228 
112 
no 

78 
36,38 

34 
192 
188 
166 
166 
167 
170 
169 

175 
168 

50 
193 
194 
194 
197 
180 
204 
205 
206 

206 
207 
207 
189 
126 
121 
123 


INDEX.  265 


^ass,  (or  embedding             ......  n 

Welch's  method  of  demonstrating  subcutaneous  connective-tissue 

cells            .........  35 

Yellow  elastic  tissue          .......  30 

Zonula  ciliaris                      .......  234 


MANN.  A  Manual  of  Prescription  Writing.  By  Mat- 
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